HOWEVER (and it is a big HOWEVER), it is more than a little problematic, IMHO, that anyone should claim that the GM should adjudicate when non-codified exceptions should be made only when those exceptions do not affect the PCs. I have no interest in such game play.
YMMV, obviously, and obviously some people's M does V. However, the DM maintains that right and responsibility in 4e, per RAW, and that is as important a part of the RAW as the part that says a creature is aware of an effect. The big problem here isn't the exception-based design, but the vocal minority who claim that the GM should not adjudicate when it affects their characters in a negative way.
Well, since I have no interest in such game play, either, I'm certainly not in the group that claims that the DM should not so adjudicate...
I do think that the DM needs to be:
1. Consistent. To the extent that you squash PC abilities for other concerns, you need to squash monster abilities in a comparable way (keeping in mind that "comparable" is not a straight mechanical question, either, since the PCs are front and center all the time.) This is one of the big reasons why my gaming style on such rulings is that I want consistency more than any particular ruling, and why I let the players vote for how we go if there is disagreement. Then I enforce the decision, hard. That vote gives me, in some ways, more power than the Viking Hat ever did.
2. 4E has narrative conceits (if not Forge Narrative mechanics) that are metagaming. I think you really don't get very good 4E play if you refuse to engage those conceits, anymore than you get, say, good basketball sitting down. You can find exceptions (e.g. wheelchair basketball) for various niche reasons, but for most people, no.
There is a certain sussing out what a game does, and how it wants to do it. Once you have it, you can buck the game or you can go with the flow. Or, of course, decide it isn't for you. But I think bucking the game and then not being very happy with the outcome doesn't say much about what that game is capable of producing.