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What do you think would be the reason that it wouldn't fly?As I said: intent, which would definitely be scrutinized in a case where you're up in front of a judge. There's some interestingness in determining whether the ToS is actionable or not (and in some countries it isn't), but I find it really hard to argue legally or morally that Youtube can't mandate that you don't hit their service with a tool whose only purpose it is to copy videos.
It's a bit more murky when dealing with a tool like youtube-dl which is a program you run on your own machine, but pretty cut and dry for a web service you're providing to others.
>why wouldn't authorization be implied when the server responds, regardless even of what the TOS says? Also the TOS seems a bit ambiguous, but my main defense against that would be to send my own TOS to the server on every request.
They're providing the service, you are not. If you don't like their service's requirements, you are free to not use it. The logical conclusion of this hypothetical world is that anyone sending the X-User-TOS header would simply be met with a 403. Most other companies would follow suit, since it's absurd for a service that serves millions of users daily to comply with the legal demands of all of them.
Worse, nothing has been gained in this world. Just as YouTube is free to demand you use the website or not at all, they are free to reject clients who demand extra obligations. Their back yard, their rules at the end of the day.