No one has argued Youtube isn't allowed to do it. It is just customers complaining about the move. If McDonalds got rid of their Big Macs, people might complain about it, but that doesn't mean that McDonalds has a hamburger monopoly or that the complainers think McDonalds did something illegal or something they weren't allowed to do. It is just customers complaining about a feature they liked being removed.
If you go to the store, demand a Big Mac, and complain to all present that the Big Mac got cancelled, then they'd ask you to leave. And, sure, folks would be upset about being asked to leave.
