YouTube is not publicly accessible: they offer it under specific terms, namely that you watch ads or pay directly. If you don’t want to accept that deal you don’t have any more right to use their private property than you do to complain that the local movie theater wouldn’t let you sneak in when you didn’t like their ticket prices.
That doesn’t mean it’s allowed, only that they haven’t blocked it again. You can’t use your neighbors’ pool without permission just because they don’t have a guard checking IDs, either.
