Whether S&P says it or not, then, we should consider part of the 'default' risk being that USGov intentionally, strategically breaks those stated inflation assumptions.
If there is a risk that the US will significantly inflate the currency, then that risk should be reflected in the ratings for all dollar-denominated bonds, not just Treasuries. But there are still AAA-rated corporate bonds. Heck, S&P still rates the bonds of 13 states as AAA, which makes no sense to me.
