Is it that big an advantage to test APIs that are still in beta and may change at any time? With their built in apps they can fix both pieces at once, but if they started breaking large numbers of apps every update I do not think developers would be happy.
IMO they should be required to offer the same API they use when they release an app that's using it. I don't think it has to be stable - it just has to be at the level of maturity that their internal apps are using. That does require a certain minimal level of security and inspectability - but that seems reasonable when you're selling a platform like this.
The Google acquisition of Fitbit was only approved by the EU under the stipulation that they can't offer any 1st party APIs for Fitbit/Google wearables that aren't available to 3rd party wearables.
Apple Watch doesn't have to play by the same rules.