Breaking peoples product is bad enough, but not taking the time to make a clean announcement such as on their official blog seems unprofessional to me. Hence the "quietly".
FTA:
> the notice came in the form of an updated Frequently Asked Questions page outlining the termination, and an email to developers active on the platform.
Any developers using the API in their product would've received an email.
It's possible they do not see any significant use. It's also possible they contacted some developers directly.
You are making some assumptions here to reach the conclusion they are acting unprofessionally.
Seems to cover anyone with an interest in the API more effectively than posting on any of Google's many blogs would.