The other thing is as you mentioned for maybe getting an reward in any kind of form for reporting that this could be abused from others.
Creating an new Account and spam with that and with the main Account you reporting it.
But the idea itself is interesting.
If there is a reward, it will be based on the report accuracy percentage as well as the number of reported posts, preventing abuse from random accounts reporting thousands of posts in an attempt to outwit the system.
There are users with thousands of reported cases yet their accuracy rate is less than 70%.