Why would S&P do this? Well, if you remember the Abu Dhabi/Kings County fraud case, S&P and Moody's argued that they were not, in fact, committing fraud when they rated dog shit as investment grade, but were actually engaging in free speech.
What better way to demonstrate that the ratings you peddle are as worthless as, let's say, a random blogger's angry post than to call out the institution which helped bail out your entire industry with a laughably unjustified and ineffective downgrade? No one's going to listen to you--and they haven't--and you'll get to use it as evidence of the free speech nature of your business in any ongoing litigation.
Fucking 11-dimensional chess players, these guys.