I don't really mind if a visiting user creates a bounty, gets their answer and leaves. IMHO that's one of the most useful benefits of the free rep (the other being the ability to comment1).
If I'm not normally a user of something like English Language & Usage, for example, and I did have a question about English, and it didn't get enough attention, I'd like to be able to award a bounty on it.
It is, after all, rewarding my activity elsewhere (while possibly rewarding someone's activity on ELU). It's not as if I can get back the rep, or get it up to 100 again without participating in the site.
(I guess ELU is a poor choice for an example, since I believe I could contribute good quality content to ELU. Information Security, on the other hand, is an excellent example of where a newbie like me could do real damage with terrible advice if not downvoted in time and a place where I'd normally not answer.)
1(Actually, that ability to comment everywhere was the initial driving factor behind my activity on AU.)
