I have to admit that I was also wondering about the anti-trust aspects of this. Given that YouTube is a dominant player (73.1% market share in the US according to [1], probably higher in many European countries where there are fewer alternative players on the market), one could argue that Google is abusing their market power in Internet video to push their own mobile OS. Since Google does not offer an app for Windows Phone, their YouTube Red service is basically 9.99$ for Android users and 12.99$ for all other supported mobile platforms. And - as others have pointed out - the 30% cut for Apple is not mandatory (see Netflix et al).
But YouTube is usable on WinPho using IE though, isn't it? Sure it's not a first class experience, but talking as someone with a fair bit of WinPho experience...
What on WinPho is a first class experience?
(Admittedly I've never actually tried YouTube in IE on WinPho, but the 3rd party MetroTube app is supposed to be good.)
[1] http://www.statista.com/statistics/266201/us-market-share-of...