The problem is that nobody goes after manufacturers that violate the GPL. If Google were to put their money where their mouth is, they should pursue ALL the manufacturers that refuse to release the GPL code to their Android software.
Here are some of the big GPL violators:
What would that accomplish? The only thing that you could get is whatever kernel modifications they've made. Do you really think there's a lot of really innovative kernel work being done by those players? And, AFAIK, they do publish the kernel changes to comply with the GPL. Samsung and HTC do, anyway. I'm not sure about the smaller ones.
The rest of Android is under the Apache2 license, so OEMs have no obligation to publish their changes. Not even to Google.