Apple has always made services primarily for the users of their hardware. They are a hardware company that makes their own software and services. The hardware purchase funds the software development.
Who decides which platforms get support, if it's not the company making and supporting the service? When BBM was popular, I know a lot of people without Blackberries would have liked to use it, but Blackberry didn't offer it, and no one was threatening legal action against them (that I know of). I don't see how this situation is any different.
There are a lot of exclusive services out there, which are locked to specific platforms. Affording legal protection to anyone who hacks their way into a system, and telling the company they can't do anything about it, would create chaos in the tech world. There might be some cool projects, but business models would fall apart, companies would fail, security would be worse than it already is, and I'd question why anyone would try and start something new when they wouldn't be allowed to control it in a way to ensure profitability. Having everything free and open is great, but at some point these services need to be paid for.