If it were spam I think they'd tell them that. If they don't tell you the reason, almost certainly its fraud, and the only way to win against fraudsters is not

If it were spam I think they'd tell them that. If they don't tell you the reason, almost certainly its fraud, and the only way to win against fraudsters is not to play with them. Anything you tell them is just an invitation for them to argue with you and waste your time, or infer detection methods. It is very hard to control fraud costs without alienating your users, especially for a free service with low barriers to entry. That isn't an excuse for Google to be so terrible, but I do think people lack appreciation for the actual problems here.