I don't really see what the Pixel-only early API releases achieve except for allowing developers to work on Pixels ahead of time, but Pixels are such a small sliver of the universe that it basically just gives Google a leg up, I would assume. And if you're on Graphene why would you care about Google's beta edge apps?
Exclusive access to QPR1 and QPR3 releases gives Pixels an unfair advantage over other Android OEMs. They get an extra 2 major updates per year. Introducing new APIs for third party app developers as part of these updates means third party apps will now run best on the Pixel OS. Google apps already run best on the Pixel OS due to many exclusive features. It's Google's standard overall approach to propping up parts of their business with their monopolies in other markets. It's not legal.