Kinda like religion. You have the same on the other side, but to gain power instead of the likeliness of the rest.
My country's army publishes statistics on who they had to kick out over the previous year; there do manage to be some people who get discharged for being left-wing nutters, but currently they (as well as those who are discharged for being radical islamist nutters) are far outnumbered by those who get discharged for being right-wing nutters.