Couple things... first they have other methods of tracking down what you own. Second, they don't know who everyone on here is and would have a hard time knowing without a lot of investigation.
Also, if he is BATF, shunning him like that and accusing him of trying to find out what we have so he can take it probably would only reenforce his belief that we are indeed crazy gun nuts like Janet keeps telling him and that taking our guns is the right thing to do. I can understand hinting that he may be, but then again I think most people who read these forums are smart enough to realize that, but may post pictures anyway.