Well it's not quite as dramatic as that.
If you obtain a certificate of posting when you send your game back, then they will charge you nothing and probably nothing on the first couple of discs anyway. But if your discs regularly get lost and you don't have the certificate, then they can charge you the RRP of the game automatically. It's what you agree to when you sign up with them.
I got fed up of traipsing 2 miles to the post office to get this certificate when there's a post box just 100 yards up the road.