I would be upset because I bought the hardware and they arbitrarily won't let my music from Spotify play on it while their own music does.

Remember it's my device, but theirs. Not apples. If they are going to allow third party apps then do so, but there's no technical reason to not allow the service.

Imagine Ford says you can only use a certain brand of tires on your new car, or can only buy tires through the dealership. That's what's going on here.