Huh? Apple doesn’t stop pebble from creating its own phone and OS.
Smartwatches are a market in and of themselves. Regulators know this which is why Apple has all of the antitrust cases that they do.
The idea that if you want to compete with the Apple Watch that you would need to build entire computing platform is utterly absurd, again regulators know this.
Even Microsoft, one of the worlds biggest companies, couldn't compete in mobile there just isn't a market for a third platform.