How is that any of the customer's business. For all Joe Customer knows they license the rights from the movie publishers.
But the problem is what if one of the customers happens to work for Fox and happens to notice the Spiderman DVD he got from Netflix doesn't match the ones in the posters around the office? What if Fox deliberately has employees renting from various services looking for this kind of thing? Knowing that the publishers have been wanting to put the renters out of business since day one it's just not worth the risk for the renter to do anything that violates the copy right.