Why does a question still have the:
start a bounty
field lit up, as the question has been marked as answered?
There is a blog post which describes in depth how the bounty system works. In essence, the answer to your question is that
all bounties are completely independent of and unrelated to accepting an answer.
The idea is to get people to have another look at a question if it hasn't had enough attention. Even if the original poster accepted an answer as "good enough".
See also these rules for the rewarding of the bounty from the FAQ:
- If you do not award your bounty within 7 days (plus the grace period), the highest voted answer created after the bounty started with at least 2 upvotes will be awarded half the bounty amount. If there's no answer meeting that criteria, the bounty is not awarded to anyone.
- If the bounty was started by the question owner, and the question owner accepts an answer during the bounty period, and the bounty expires without an explicit award – we assume the bounty owner liked the answer they accepted and award it the full bounty amount at the time of bounty expiration.