[turtle4]

If nothing is given, you assume the first party is indicating that their thanks/recognition is enough.

If something is given, then they are acknowledging that their thanks/recognition is not enough, but that $x monetary value makes up the difference.

Some people take it as an insult that if thanks isn't enough, they can be 'bought' for $x, where $x is $12.50, or whatever. The reasoning is basically that if you are saying the time taken to reproduce and report the issue is important enough to compensate, you should probably compensate commensurate with what they would expect to make for their time/expertise.

I think most people who report these things do so because they want to improve things, not for the money, so most probably fall into the first camp, but I can understand where the second camp is coming from.