Because they are a monopoly and acting both as a publisher and as a platform. If they are a platform, then they are not responsible for what people post on there, and so shouldn't censor. If they do censor, they are acting as a publisher, which removes their "platform" status of no responsibility for what the users post. Right now they are acting very much like the USSR's state media, and that's not something US law is theoretically acceptive of.