It's a good thing that green energy isn't a fixed quantity and more can be built if there's demand, which is exactly what Google is doing.

It's great for marketing and, you're right, that's why they are doing it but the reason it's good for marketing is that their customers care about the environment. This is the exact invisible hand mechanism in action. It's their preferences changing the behavior of one of the biggest corporations in the world and all they can do is act like it doesn't count because they don't actually care