Apple wrote the application, and runs the servers on the back end to support it. Why shouldn’t they be allowed to choose where they should port that software?

Apple wrote the application, and runs the servers on the back end to support it. Why shouldn’t they be allowed to choose where they should port that software? Should they be forced to port it to Windows? Should they be forced to port it to Linux? Maybe they should be forced to port it to plain old analog telephone systems?