I don't work for Google, so my answer will just be a guess at best.
Let's look at this button in basic terms:
download bmml source – Wireframes created with Balsamiq Mockups
It's necessary to make clear that the state is active. Google does this by changing the colour. With this type of button, you would expect that clicking again would deactivate it.
The other reason is probably more important: Google doesn't want to take up much space here to explain what's going on, or going to happen next.
Imagine the button after clicking it would change to something stating the next action when clicking it again, with no more space used to explain the context:
download bmml source
Hang on, what just happened there? Did I
-1? What happens when I click on it? Can I go to negative by clicking it repeatedly (and start a flamewar)?