Except, why is there a law to have a plug in your auto-5?
Fish & Game regulations typically limit the number of rounds you may carry loaded in your hunting firearm - since you have a "bag limit" - a restriction on the number of game animals you can legally take - there's no reason to have 30 rounds of ammo in your firearm unless you're poaching.
A lot of the regulations are like this - since they can't read your mind and know whether your intent is poaching or legal hunting, and since they usually can't catch anyone red-handed given the nature of the wilderness, they come up with concrete, specific restrictions that can be determined in a legalistic, black-and-white fashion, such as whether you have a plug in your magazine limiting the number of rounds it can accept.