This is because the Basel accords are a product of the (evolving) financial conventional wisdom of the last 20 years. According to Neoclassical Economics, ratings agencies make a lot of sense!

This ECB rule is the same mistake writ large.

But this is an ECB internal rule, right? Not a consequence of the treaties. What A central bank will accept as collateral at the discount window is entirely up to that central bank, and in fact that choice is a key instrument of both banking regulation and monetary policy.

Of course, this assumes the ECB is not blindly following Monetarist dogma... Oh, wait!

The brainless should not be in banking -- Willem Buiter