I assume that YouTube's licenses with the RIAA covers the Youtube Premium features on their videos. If they didn't, they'd be in violation of RIAA's copyright (streaming licenses are not the same as downloading ones).
You know... I read the Youtube license and I see this:
Licence to Other Users
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.
I am a user of the Youtube service. I read and agreed with the terms, I have watched videos.
Thu I am allowed to reproduce its content. The RIAA explicitly agreed I can distribute it.
I'm not a youtube user, and I didn't agree to their terms of service. I can however plug their URL's into a terminal without actually accessing their web site
"Spitting out an mp4 file that you can copy to another device" is not a service that YouTube provides. That's why youtube-dl exists. If it was a service that YouTube already provided then there would not be as much reason for youtube-dl to exist.
"Unenforceable" doesn't mean practically unenforceable, it means legally unenforceable. The laws of physics permit most cars to exceed most speed limits. Those laws are still enforceable.
Unenforceable clauses in contracts are things that are either contrary to law or otherwise don't fall under contract law. So a clause that says "if you sign this you must give up your firstborn son" is unenforceable. That sentence just says that "if we screwed up drafting this contract, just because you were able to challenge one clause due to being unenforceable doesn't mean the whole contract is void." It's a severability clause.
However, Safari (not Firefox) is not affiliated with Google. Google has no say on how the content provided by the Service is presented. The Service provides a HTTPS connection. Its influences ends there.
