S&P is a ratings agency.
As a private entity, its valuations (sold to investors) are its main cash cow. You don't have to believe what they say--that's your prerogative. If you think what they say is BS, out of touch with reality, or a "$X trillion mistake," that's what the other rating agencies are for (Fitch, Moody's, etc.).
Who gave S&P the authority? Private enterprise gave them that. They happen to be very good at gauging these things, which is why people tend to trust them.
OK, so they were manipulated (or manipulating) for mortgage pass-through securities during the 2008 credit crisis. They fell asleep at the wheel, should have been paying more attention, should have been better at figuring it out. They could have been more savvy to realize that the CDS market was getting bigger and bigger all at AIG's expense. But they weren't. That also happens to NOT be their primary market. Fannie and Freddie are supposed to be responsible for securing mortgages. S&P is all about stocks, bonds, and indices.
Think of how many times does S&P get it right compared to how many times it is off base. If there is one plane crash, does everyone decide not to fly ever again?
I'm no big fan of S&P, but at the end of the day, S&P is answerable to its investors and its clients, no one else. Certainly not Robert Reich. Who gave HIM the authority?