Because you don't own it.
What you say is true but you seemed to have taken my post out of context.
I was referring to the content of the disc where there is, in fact, no difference.
I refuse to help people with rental disks because I don't believe in copying rental disks, that is stealing.That's not always true. But more importantly, having to help people with greasy, scratched rental discs is not worth my time (besides the fact that I can't help with rental discs).