I noticed that too. Most of these questions are around a 50-100 points bounty. That means, that are worth about 2-3 answers (or one really good one). In most cases that I've seen, they then tend to want a lot - close to a finished plugin and similar - but don't get it. People can do basic math (at least most of them) and 15 points for a solution + 10 for an upvote = worth half of the bounty.
EDIT Here's the reputation history of a user, who is only gaining reputation to use it as do my task for my-bounty.
I personally don't really care about this, as the amount of such bounties is still low. If you look at the featured section on SO, then this might be a problem in a year or two. But then we're maybe at a point, where simply raising the min. bounty is enough.
EDIT 2 As I stumbled upon another user who uses bounties to get people to code close to finished plugins for him, I'm not too sure anymore if we're handling this issue correct...