> if they have the ability to detect and block fraud, then why block accounts?

To protect their business by protecting their reputation.

The scale of Google's advertising business(es) means that a loss in revenue from poor public perception of their ads is likely to be far more than a few thousand small ad buyers (those spending <US$5000/mo) getting cut off, which would barely register as a rounding error (<0.1% of just AdWords revenue alone.)