I used to work for a company that sold DVD's etc.
There were two versions of most films available, Rental Only (which cost from about �45 each) and Not For Rental (which cost about �13). The not for rental versions are clearly marked on the box and the actual disc.
The rental versions were only available for a certain time as only a limited number were made and once the stocks run out, that's usually it.
Ocassionally we could 'lose' a customer order for a major title, and by the time they discovered that they were missing a new release, it was impossible to get them a copy as the Rental Only versions were now unavailable and they were not allowed to rent out the 'not for rental' version under any circumstances. Same as if a disc got snapped or wasn't returned by a customer, often it couldn't be replaced as the film was now unavailable in the rental only format.