There are no rules with regards to giving merits as long as it's constructive, meaningful or helpful to other users. It is because merits/merit sources are scarce that they have to assess who to give their merits to thus insert deserving high-quality post.
If there are rules about it is there any consequences to the violators? Because i think that there where some people who gives merit even in low quality post.
Low-quality posts should not be given merits freely, if in those cases, it is possible that users are giving merits to their alt accounts to just gain merits. This also leads to merit distribution abuse which you can investigate if there's an exchange of merits between people, etc. But with that method, it will not really last long as Smerits decreases by half everytime you send one. Also, those users must be put in the watchlist as there's a possibility of merit abuse.