Not at all - I'm not saying that dumb cutpurse NPCs should be able to deduce what the PC is up to. What I'm trying to say is that if in a fantasy world, a character became notorious for
always using the same tactic, the NPCs would eventually learn who he is, and know what his tactic is. Let's say the DM who started this whole thread was lax and let his PC use this tactic well into the higher levels. If this is the only trick in his deck (despite high-level abilities), he's eventually going to encounter a powerful NPC (who may be dumb as dirt, but powerful) and be well aware of what the character's general tactic is. Yes, it is metagaming on the part of the DM to set up such an encounter, but I for one feel it is necessary, when you have the PCs overly relying on something.
I've been gaming for 20-odd years, and it's generally been accepted that if a party becomes too reliant on anything - an item, ability, skill, etc. - you find a way to present a challenge to the PCs that will cause them to...gasp...get creative.
Don't get me wrong - I see the anti-metagaming argument that you and many others have presented, and I believe it IS valid. But, this character's one-trick arsenal is obviously ruining the fun of the game (At least for the DM...)