The artists sign a contract with youtube. Non subscribed users don't. The same way promises only apply to those who believe in them, terms of services only applies to people signing a contract for them.

If a physical shop decide their terms only allow the customers with a membership card to look at the window from outside but decide to not put curtains, those terms do not apply to bystanders who could see inside. Same when my neighbour is naked with all the lights on. If she doesn't want me to see her, she knows what to do.