Being able to publicly claim your product as the one that underlies part of the federal reserve banking system has got to be worth some real money to IBM.
That's part of it I'm sure, but it's much more than just advertising. What bank is going to make a risky move to a new message queueing service to save a few bucks when all their connections with the Fed use IBM's? And the hook there goes even deeper, because all of this stuff is deeply integrated with your databases.