For instance, if I receive a HTML/CSS data that displays a trademarked design (which should be kept as a whole), I am able to mangle the presentation part any way I want for personal usage. If I publish a site that's a mangled copy of a trademarked site, that's gray area. For my PC usage, no gray area. Same with disassembling, reverse engineering, etc.
Then, I have a right to publish any tools that perform any customizations over owner's items, be them hardware or software. Customizing proprietary software and reselling it is illicit. Customizing for one's own purposes isn't (the proprietor may choose to drop support for owner on his prerogative).
Most important point is that YT and alikes want to freeload in the middle area. They want to be free and public, but they want to force the way you consume them. That is a no go. YT can choose to make a closed system, but then they would lose all random people without accounts finding YT links and watching them. Really, tie the google account, and set up watching quotas, direct payment or any other system where YT directly gets "paid back". The user can use his tokens to access streams from wherever he wants, youtube-dl or whatnot.