They're relying on the manufacturers not changing their tooling / marketing. Aren't they?
When you try to ban products by make/model, then yes, that's exactly what they're relying on. So long as only one state is playing that game, it can be effective because it's not generally economical to make a new but slightly-different product for that one state. But when many states start trying to play that game, it could become worthwhile to start putting out "new" models on a regular basis.
"Hey! What year AR is that?"
"Oh, it's a 2015."
Yeah, we'd lose out on parts-commonality, which is one of the big draws of the AR and AK platforms, but it's better than losing the entire platform.