Bounties are considered special.
If users were able to place trivial amounts of reputation for bounties, everyone would to it in order to seek attention for their questions. This in turn would render the concept of bounties useless — imagine a place where every unanswered question had a little bounty on it …
I personally wouldn't like that, and it would even discourage users from looking at "featured" questions if there were too many of them, with too little incentive for answering those.
Remember that everyone here works hard for their reputation, so you need to earn the privilege to place a bounty first. And don't forget, if you post a really good question, it's easy to get back a little reputation from upvotes on it later, once you place a bounty.