Moderation on the basis of ideological purity does not produce "quality content," just an artificial ideological uniformity.
People have a tendency to hide behind the principle of free speech as if it were all-encompassing and omnibenevolent. It isn't. Just because you have the right to speak your mind doesn't mean you have the right to be heard. Low-quality contributions can and must be suppressed if the level of discussion in a community is to remain high.