Take the challenge from their side. If they leave the bb-gun sub active, how do they ensure that people aren't going to the bb-gun sub, and trading real guns there? They'd have to have humans police each post, or develop ML algorithms that can reliably identify when someone is hiding a real gun sale as a bb-gun sale. The former doesn't scale reliably (sure, the mods of the bb-gun sub could probably do it voluntarily, but I doubt reddit wants to bet the corporation on their continuing to do so), and the latter doesn't yet exist.

Banning both subs definitely stops the illegal transactions at the (relatively minor) cost of not letting people trade bb-guns on their site.