I suppose 37 Signals and Adobe are ripping off their customers as well because their marginal cost is close to zero and they are charging... more than zero.

Actually, I agree SMS is a ripoff, but the marginal cost isn't why. It's a ripoff because it's something people want and there is no other way for them to get it, which is an effective monopoly. With Google Voice and other competitors gaining adoption on smarter phones, I expect this to change.