Cannot legally back up depends heavily on the specifics. For a long time (IDK about currently) it was legal to rip your own discs for backup purposes, and even to break the encryption as long as you did it yourself. So it was potentially legal but not necessarily within reach to do it legally. And that is, of course, wrong. There should never be any uncertainty about whether you can back up the media you purchased.

Saying you're "renting" the disc is just plain disingenuous, though. You own it free and clear. You can give it to anyone you like, for free or for a profit (i.e. how Netflix did things back in the day). The original owner is not asserting the right to come and take it back from you. It is in no way a rental, it is a sale.