It strikes me that the most popular reason for starting a bounty on a question is because the question hasn't received any answers thus far. However, when starting a bounty, whether there are any answers or not, it still presents you with the full array of reasons for starting the bounty.
Three of the reasons should surely be removed in this instance:
- Current answers are outdated
- Improve details
- Reward existing answer
If the screen is still shown I'd advocate that they were greyed out rather than removed for the sake of consistency.
However, in truth, I think I'd go one step further and suggest this entire screen is skipped in the case where there are no answers. Surely in that case, it's pretty obvious that the reason you're starting a bounty is just "there's no damn answers and I need an answer!" (Or, out of the current options, perhaps that should be "draw attention".)