Bugs found by your paying customers have a cost even if they appear to you to be free. They increase the total cost of using your software, and in a perfectly fungible world ruled by rationality that's extra money you could be charging them for bug free software.

It also helps with massively distributed software where invidiuals are less likely to report bugs (without some kind of automated bug sendoff tool). Netscape had their $50 bug scheme, for instance.