In the case of copyright you are not buying the media. You are buying a licensed right sent as media. If they wish to make no impediments to making a backup then maybe they have no fault.
Warranty is about fitness for use, merchantability, and honoring sales contracts. It has nothing to do with issues of intellectual property.
If you sold someone a book, filled with public domain content, and the ink faded after a week, you can't just throw up your hands and say that the buyer should have made another copy. You'd likely be liable for warranty of merchantability.
The fact that it's a limited license attached to the physical disc only makes my case more clear, I think. There's even less expectation that I have some claim to the content if the disc breaks in 20 years.
It was only with the advent of toner based xerox in the 70s that photocopying books became vaguely practical, but still cost about 5 cents a page, at a time when a mass market paperback might cost $1.25.
The US didn't correctly defend the right to backup, but that leaves a funny grey zone where WB being bad at making DVDs means less ability to shame "piracy" tool makers because backups is normalized and in no way suspicious if the media isn't perfect like they said.