I believe it is a security thing. It has been designated as a checked bag once the tag is on it, and checked bags can have things in them that carry ons cant, ie your shampoo that is considered a high grade explosive because it is over 3 ounces. "They" don't know if you curb side checked it with say, a gun in it, and then someone brought it up to you, and you are going to cause trouble.

I agree that it's dumb from a gate check stand point, but you know the FAA likes their legalese.