Promising such rewards does not contradict Allah's justice and doesn’t negate the correspondence between acts and rewards; because if justice means to place everything in its appropriate position, which means that there should be a correspondence between acts and rewards, regarding the examples mentioned, there is complete correspondence because:
First: These ahadith are stressing the value and importance of that act of worship, not understating the value of hajj and jihad; in fact these ahadith indirectly show the greatness of acts like jihad and Hajj because they are considered the standards and criteria by which all others acts are valued, the same way we would say that a certain object is as valuable as gold.
Second: These ahadith refer to situations in which it is not obligatory to perform jihad or Hajj.
Third: When observing the mentioned correspondence, we must not only consider the difficulty and physical greatness as the sole criteria by which its reward is determined. In fact the effect of intention, humility, belief and ma'refat (spiritual knowledge) on the reward can be much greater compared to an act’s physical difficulty and greatness; therefore if a certain act possesses the mentioned qualities, then it can be compared to great acts like jihad or Hajj.
Moreover, some duas and ziyarats, due to the spiritual content they consist of, can have a specific effect on one’s beliefs and faith that makes them greater than other acts which do not have such influence.
We must keep in mind that when there is no war or jihad and it is not time to perform Hajj, usually there is a greater opportunity for Satan and his allies to tempt and deceive the believer, and add to that the fact that the human instinct is inclined to satisfying its carnal desires. In such a situation for one to keep his heart and soul pure through reciting duas, ziyarats and prayers, he must possess great ma'refat and ikhlas, and therefore he might be greater than the person who actually takes part in jihad or performs Hajj.
All of this was based on justice meaning to place everything in its appropriate position, but if the meaning of justice is to not oppress or take away the rights of others, still this would not be an example of injustice; because even if Allah granted a reward that does not match the act, he has not violated the right of anyone, because setting a reward and thawab is a means of Allah's grace, not the right of a servant.
Through mentioning a few points we will explain the correspondence between the abovementioned acts and their rewards:
1- Divine rewards are not granted by Allah as an obligation towards His servants, but as a means of His grace that is bestowed on his servants depending on the acts and virtues they have accumulated.
There is no obligation for Allah to reward his servants for their deeds (if he had not promised them such a thing) because all of what they do is done by employing the very blessings he has granted; in fact it is out of his mercy and grace that he promises his servants rewards and has bound himself to keep his promise. Therefore it is his unlimited mercy and grace that determines the reward of different acts, not the difficulty or easiness nor the greatness or littleness of an act. In this light, neither the greatness nor the littleness of the blessings and rewards Allah grants his servants will contradict divine justice, because as we mentioned before, no right has been violated and in fact the servants have been given more than they deserve.
2- Duas, ziyarats and mustahab prayers will only have such great rewards when performed along with their special conditions, some of which are mentioned below:
A: Performing all obligatory and wajib duties: Therefore if a person has an obligation such as jihad or Hajj and instead spends his time reciting duas and performing mustahab prayers, not only will he not gain any thawab, but he might even be punished by Allah.
B: Having presence of heart, ma'refat and ikhlas. In this regard, it is mentioned in some ahadith that the condition of reaching the great reward of some ziyarats is to possess ma'refat. In a hadith by Imam Sadiq (as), he says: "People will be 'Mahshoor' (resurrected) according to their intentions on the Day of Judgment." This hadith clearly shows that the true criteria by which a reward is determined is not the difficulty of the act. Instead the intention, belief and ma'refat of the believer have the greater effect on the reward than its physical greatness and difficulty.
Obviously, if possessing true ma'refat, complete ikhlas and presence of heart is not more difficult than taking part in jihad or Hajj, it is definitely not easier. And this is exactly why many people may perform Hajj or jihad, but very little people truly reach the great levels of ikhlas and ma'refat. The Ma'sums have encouraged Shias through many ahadith to perform ziyarat of the Imams and especially Imam Reza, it is also understood from these ahadith that the ziyarat can have two different rewards depending on the way the ziyarat is performed. Therefore there is not a similar reward for all ziyarats. Also what is mentioned in these ahadith is the potential reward, not that believers will gain the mentioned reward under all circumstances.
3- Many of the duas and ziyarats play a crucial role of explaining theological issues and spreading ma'refat and knowledge among people, and the imams have taught and transmitted to us truths and knowledge through duas and ziyarats that they couldn’t have conveyed in any other form. Therefore paying attention to them is a way of protecting jihad, the culture of shahadah (martyrdom), hajj, and the shariah. In other words it is the ma'refat and truths that create mujahids and hajj pilgrims.
4- When there is no war or jihad and it is not time to perform hajj, usually there is a greater opportunity for Satan and his allies to tempt and deceive the believer, and they are strengthened by the fact that the human instinct is inclined to satisfying its carnal desires. In such a situation, for one to keep his heart and soul pure through reciting duas, ziyarats and prayers, he must have great ma'refat and ikhlas. This is why the Prophet of Islam told a group of his followers returning from an exhausting war: "Praise is due to those who returned from the small war while the great jihad lies ahead of them." The companions asked what he meant. The prophet responded: "(The great jihad is) the jihad against the self."
And maybe one of the reasons why the Imams have mentioned such rewards for the duas and prayers was to prevent society from being caught in the trap of physical and carnal desires, which entail misery in this world and the hereafter, so that it can fly in the sky of spirituality and prosperity.
In any event, these ahadith are stressing the value and importance of that act of worship, not understating the value of Hajj and jihad; in fact these ahadith indirectly show the greatness of acts like jihad and hajj because they are considered the standards and criteria by which others acts are valued, the same way we would say that a certain object is as valuable as gold.