In other words, OEM WinPhone users are already cornered into producing only the stock vanilla experience, and now they're competing against the platform owners themselves who are allowed to break whatever rules necessary to gain market share.
All on a platform that wasn't doing well to begin with - I can't imagine a sane OEM wanting to stick around. Maybe if WinPhone was selling like hotcakes you might expect OEMs to just bear it, but as it is MS in a poor place.
There is also another key difference to Nexus: the Nexus phones have never been heavily marketed, and even till now are the tools of developers and industry enthusiasts. I imagine if Google ever took the Nexus mass-market they'd see much more blowback from OEMs.