I think I can come up with a theory on why you don’t want to pay for grades.
It seems to me that getting good grades is in the kids own self-interest and it seems to me that paying kids to do stuff that is in their own best interest anyway isn’t such a good idea.
If they have to learn to do things that are in their own best interest by not doing them and suffering the consequences, then so be it.
I’m adding that to my list of parenting concepts – “don’t pay kids to do things that are in their own self interest.”