Third-party controllers have been widely available for every popular video game console including the GameCube. Nothing was stopping anyone - Microsoft included - from creating an Xbox controller which would also work on the GameCube. Nintendo certainly didn't stop anyone.

This example completely misrepresents the issue. Nobody is asking Apple to add support for Pebble watches to their devices. They're asking Apple to stop preventing Pebble (and other smart watch manufacturers) from being able to support Apple devices.