If the U.S. falls back into recession we'll take everyone with us. Other countries know this. Banks know this. Institutional Investors know this.
Increase the interest rate on the U.S. and you'll trigger an increase on the U.S. consumer while exacerbating the U.S. debt crisis (which is the whole reason for the downgrade). That would guarantee a recession and would bring every investment down.
Finally everyone knows what S&P is up to. You said it yourself. They aren't happy with U.S. policy and they've been able to bully other governments into austerity in the past. So they think they can do it here. But S&P's opinion of U.S. policy isn't going to be enough to convince investors to willingly create another recession
(Please note I'm in favor of austerity and think we're already headed into a recession I'm just realistic about S&P's limited power)