I think there are cases when the bounty placer just forgets or can't award the bounty (out of email/internet etc.), but it's perfectly justifiable to award 100% of the bounty automatically for him.
If the bounty placer accepts an answer within the time-frame of the bounty and doesn't award the bounty in time, award 100% of the bounty automatically to this answer, regardless of the number of upvotes, in the name of the bounty placer (not Community).
I think that accepting an answer is enough to say - "This is what I wanted!". Then he just keeps the bounty open (once he paid for the attention, why would he give up earlier), but might not be able to come back to it in time.