Not sure about that. Some do indeed bill on a monthly cycle. Some charge the basic costs on a 6 or 12 month basis, but charge for bandwidth usage monthly.
People aren’t saying Google can do what it wants. They’re saying Google is not wrong to do what it said in advance it would do, and was agreed by the webmaster on signing up.
The message admits a mistake in cutting off the ads without giving a warning and opportunity to fix any compliance issues. That’s independent of the question of whether or not they had compliance issues in the first place.
After reading it more closely, yeah, they were noncompliant. They just didn’t get advance warning requesting that they comply before AdSense got shut off.