Technological locks aren't always about ethics. Often times it's about being practical. As much as people love bashing Apple, Apple works hard to give people a stable environment to build apps in. If you think it's not stable, then you've certainly never built apps in the jailbreak environment. Sure there are cool things you can do which you can't in Apple's sandboxed environment, but it's also the wild west. It's cool if people want to jailbreak and invent cool stuff, but expecting Apple to support it doesn't really support the community at large in a scalable way. Jailbreaking is a good thing, but most people don't really consider the business implications of it.
I would just reserve the right to refuse to service jailbroken phones (and maybe service them anyway if there is no added hassle, as a goodwill thing) instead of trying to stop people from doing it. Not supporting is very different from legally and technically limiting.
Honestly I don't think most users want to jailbreak the OS, just as most android users don't want to install other OSes. iPhone users resort to jailbreaking just so they an use their carrier of choice without restrictions.