I think that it doesn't make any sense to force me to wait three days until I can reward an existing answer. I totally see the sense of this regulation for the other cases.
This is especially bad as it may reduce the number of cases in which this happens. Now I have to remember for two days that I wanted to give a bounty instead of just throwing some rep at the answer.
Furthermore IMHO it doesn't make sense to show these questions in the bounty list. You even have to open them to see that the bounty has effectively already been awarded (just not technically yet).