In the case of facebook. If they removed one side from debating that side will leave. Creating a smaller more similiar group of people. That would go against the goal of connecting the world and change the goal to connecting the left or right. Plenty of those sites. Facebook's ideal strategy is not to get involved.
No one ever starved because they couldn't call me a f-----. Not too long ago the fact that I am a f----- would have prevented me from working in my chosen or almost any field and gotten me harassed or assaulted on a regular basis.