Their rationale was, in any case, that "entitlements spending" was too high:
https://www.theguardian.com/business/2011/aug/06/sandp-debt-...
The entire rest of the market (and the other ratings agencies) were just about able to determine that if you have the keys to the cash printer you're actually not in danger of running out of cash and political bickering doesn't translate into risk of political suicide.