It is not 100, it's 75 as mentioned in the [Set Bounties privilege page](https://meta.stackexchange.com/help/privileges/set-bounties):

>[![][1]][1]

At 50 reputation one gets the Comment Anywhere privilege, so the Set Bounties is between that and the three privileges given at 100 rep. Makes sense for me.


  [1]: https://i.sstatic.net/3gRNh.png