But that’s the thing, a bot can’t scoop it up without going through the user, without acquiring it from them in some way. Twitter are bypassing the user entirely and taking it from them. Also, a bot is illegitimate, however in selling usernames itself Twitter is effectively legitimising the practice.
Either usernames have no value, in which case Twitter can do with them as they please, or the usernames have value and that value rightfully belongs to the user that holds it.