This has to do more with the fact that physical objects wear out and digital objects don't. Publishers have complained that when a library lends a physical book, it can only do so for a limited number of times before it has to buy another copy because the first wore out. When libraries lend digital objects, they never have to buy another again. So publishers want a limit to the number of times that a digital object can be lent before requiring a repurchase. The same goes for CDs/DVDs.