They've been boiling the frog here for a long time. The "open web" is a euphemism for the Google Chrome monoculture walled-garden-in-waiting. Google exert so much influence there that building on the web is in practice barely different than building on say iOS. You can do what you want if you don't want to make money, but if you do then you will have to play along with the big G.
My hunch is Google never psychologically recovered from Facebook absolutely wiping out G+, and they have been on a mission to ensure nothing like that ever happens again.