Now that you put it that way ... When the government underwrote the bonds, THEY assumed the risk, so the rating became irrelevant. The government in essence promoted a massive fraud by committing taxpayer money to back up loans that were stupidly risky, all in the name of enfranchising people who had no business being enfranchised.
The people buying the homes thought they were the beneficiaries of this largesse, but they weren't. The ones who benefitted were: (1) the banks, and (2) the sellers of the homes.