The purpose of the Communications Decency Act is to allow providers to moderate community content, and section 230 provides liability from both undermoderating and overmoderating, so that providers don't forgo mederating altogether, for liability reasons.

If section 230 only protected from undermoderating and not from overmoderating, then providers would be incentivized to keep every comment up, from scam links to hate speech, because they'd only get in trouble for removing comments and not for allowing them.