"You also grant each other user of the Service a worldwide, non-exclusive, royalty-free licence to access your Content through the Service, and to use that Content (including to reproduce, distribute, modify, display, and perform it) only as enabled by a feature of the Service." https://www.youtube.com/static?template=terms

It is enabled by a feature of the Youtube service.

They explicitly disallow downloading, but it is not enforcable (though that word probably doesn't mean what I think it means... the law probably overrules physics)

Severance

"If it turns out that a particular term of this Agreement is not enforceable for any reason, this will not affect any other terms."