I guess the thinking is that if the developer bans an account and the slow player just creates another one, he might get paired with one of the players who already reported him. So a hassle for them.
But that's got to be balanced with the probability that the slow player is just going to keep doing it, only now with players that are new or not in the loop, so they don't realize they won't be penalized by resigning (eventually).
It would be helpful to have a clear option to BOTH ban and report. Maybe, have an option - would you like to report this player? - when you hit the ban button. As it is, it looks like slow playing, abusive language, and so on is simply tolerated by the developer and the community, and that is a big turnoff. Just my two cents. I've never had to deal with this, maybe because I almost always play bots.