Google derives their primary revenue from advertising. APIs allow developers to sidestep advertising in displaying Google's content. Ergo, it is not in Google's best interest to allow easy free access to their content, via an API or otherwise, in a place where they cannot control how the content is displayed.

I don't completely agree with this reasoning but I can see why they don't just throw build APIs everywhere.

yep. And honestly, I've been okay with google's model so far, at least until their websites and apps become littered with ads like facebook.

Aside from companies (and sociopath friends) using hootsuite, I've never really heard of anyone using a third-party facebook website or app to post.