I wouldn't be so sure about that. If Google also controls the network, they technically gain access to every bit of data you send across it, whether it originates from a Google app/service or not.
That would be the most comprehensive and effective way to datamine every last bit of information exchanged between your phone and the internet. IMHO Google would be stupid not to allow WP devices on their network.
A) They'd be seen as being open to competitor's devices, which would make them look like the good guys.
B) They'd gain access to practically any data that is relevant to them (guess who'd start listening in on your Bing searches).
C) They'd get all the data they could dream of without having to develop even a single app for WP.
D) On top of all that, they'd get WP users to pay them for those "services".
IMHO Google has nothing to lose and everything to gain by playing carrier for WP users.