Actually, if I remember correctly, in order to use 'Java' under GPL, you had to agree not to subset the API set. You could tweak and distribute, you could add and distribute, but you could not remove any portions of the Java platform that comprised the standard edition.
If you wanted to subset, you had to pay for Java Mobile Edition, and agree to carry all APIs that Sun wanted carried.
By going the Dalvik route, Google was going for the ability to control their API destiny, in the same way Microsoft attempted with their release of 'Java' in the 90's. It wasn't about copyleft concerns.