"Developmatic immunity" is hilarious and should definitely be a future quest.
I've been trying to figure out whom in my cast of characters should TR into a pure vanguard with full ****** tactics, and it appears it doesn't matter what imp. crit feat or mainhand weapon I use.
That's both a relief but also very silly that someone couldn't spec making some kind of crazy shield-only focus character. That's exactly the type of flavor that I really enjoy.
I bet it's as Festus said, and that shields were coded as some kind of separate entity just like handwraps, and thus it would take more time for them to make the system make sense than to just leave it confusing and broken as is.
That said, if they did go out of their way to make imp crit bludgeon work with shields, I don't think it would be any more pigeon-holing than any other type of synergy. People would still want to make vanguards and shield characters that use slashing or piercing weapons, and they'd just have to suck it up and except a worse crit range with their shield or invest the way anyone else has to when they want to use a different weapon system.