The idea is not to sweep them under the rug, but rather to stop the uptake of new people. A lot of people go to reddit for funny pictures, and end up being exposed to some very radical ideas. Those same people might never be drawn to a more fringe site. Banning posters doesn’t stop them from holding their views, but it seems like it may stop them from spreading. It’s a gamble, but i think it’s worked in reddit’s case