Sony's attempt to stop the resale of their games.
See, most hard core gamers I know play a game through within a few weeks of buying it and then TRADE IT IN for credit to get more games. The BluRay specs allow a disc to be registered to play on only one machine. Sony has said they are not going to implement that for PS3, however that doesn't rule out them doing it for PS4. The problem is, though, most of the hard core gamers will be turned off by not being able to trade in their finished games.
Most of the games I buy, I buy used from rental places. The rental places get a shit load of copies of the games, knowing that when it is first new, it will rent out a bunch, then they can SELL the extra copies. If they could not sell the extra copies when a game's popularity started to decline, they would not buy so many in the first place. So that would drasticaly reduce the number of copies sold of each game. Granted, they also keep talking about using some sort of DIVX-like (the works-for-four-days-only dvd format sold by Circuit City, etc., about nine or ten years back, not the video codec) discs to replace rentals. But we all know how well DIVX did and what a waste it was.