Set-top boxes may put a lid on rewritable DVDs

EETimes has an article about how cable companies want to put re-writable DVDs into future set top boxes, but the studios are trying to make them ship with a mechanism to stop them being played back in anything but the original recording device. I guess they are worried about you recording things and making them available to others for free, but a blanket ban on sharing like this will limit their usefulness to a huge extent. What if you recorded a programme on your living room box, but wanted to play it back on the one upstairs or watch it at your friends? I can uderstand doing this for "premium content", such as pay-per-view films and sporting events, but an all-encompassing limitation seems to make no sense whatsoever to me.