Because it's not GPL-derivative code? http://source.android.com/source/licenses.html

For those parts of the system that are GPL licensed, they (or the handset manufacturers who are distributing the compiled code) shouldn't be able to: http://fosspatents.blogspot.com/2011/08/most-android-vendors...

From the article, nice list of Android tablets showing those that comply with the GPL: http://www.codon.org.uk/~mjg59/android_tablets/ - a small proportion of the list are marked compliant, but most of the big names seem to be there.

Posted earlier, but not enough votes: http://news.ycombinator.com/item?id=2885787

Google doesn't have to as long as they don't distribute binaries. The liable ones are probably the phone manufacturers. But only the copyright holders can sue anyway, and I don't see that happening.