The same thing that prevents you from 'liberating' trial software, (some) demos, or retail software that requires a product key.

Not really a big deal. Although it might be difficult for someone who's never done fine soldering work, soldering 2 30 gauge wires to 2 small points on the iPhone board isn't really that hard. All you need is a good iron with small enough tips, some patience, and a little experience. I'd soldering a couple dozen iPhones to unlock them, if I could get a free car without breaking a sweat.