I think the point is in the messaging. YouTube isn't "YouTube for the left wing", it doesn't advertise itself as being hostile to certain opinions. You could argue that the terms of service denote that "offensive" content is not allowed, but something being offensive is a subjective opinion. And then there's the issue of selective enforcement of things that _are_ banned.

In other words, if a site was called "ConservativeTube" or "LiberalTube" I personally wouldn't consider it censorship for them to remove views from the opposite political wing because the service is openly politically biased.