You can't blacklist the shim, only the key it was signed with. If Microsoft accidentally signed it with (one of) their own key(s), blacklisting it would have the side-effect of also disabling everything else signed with the same key.
What else has Microsoft signed with that key? Could they practically push out updates for those components re-signed with a different key, so as to ensure that blacklisting the original key wouldn't break any Windows 8 systems? What would happen to an end-user's system if the blacklist got updated before those components?
I suppose you could see it as "childish", but I see it as not covering for Microsoft's mistakes on their behalf. They made this mess, I don't feel bad for them if they have to deal with it.