The rating agencies did not understand the products they were rating. However, the warnings were almost certainly on the label.
That's why the rating agencies were sued over this, and not the banks.
They did sue the banks. For many things including inflated appraisals of the loans.
I'm sure they have some kind of warning for everything that isn't FDIC insured. But misrepresenting high risk as low risk instead of zero risk is just a quantitative difference. There are plenty of scams that do this. for example just lying about a company's earnings. I suppose the precise crimes they charge them for may differ a bit.