And risk alienating their user base and causing increased hacking. Software differentiation really just amounts to disabled features. Buying a machine that has capabilities that you can't access pisses people off.
Would you be happy to buy a car with an air conditioning unit, but no button to turn it on? Or would you think that was a rip off. No reason that's any different with electronics.
If I bought that car, I would rewire it, and if I bought that iPhone, I would hack it.