I knew this would come up. I've even done it. But it isn't something they promote. It isn't part of YouTube itself nor emdorsed by it so you really can't make that argument. The difference I'm pointing out isnt in one's ability to download media but if the site itself was actually set up for that purpose or not.

The next argument is probably going to be something like "well YouTube should stop third party services from being able to do that" but again this is beside the point and it would be damn near impossible to stop it from happening. There's always a workaround.