They do allow it but just not many guys that do it since there are no processing plants here. Plus we have enough trouble getting a fishing line trough without getting snagged so their nets would be a bitch.
Carp are really easy to seine in ND from late May through late August. We would routinely get 20,000 lbs of them in a single 500 or 750 ft seine. FWIW, we pulled them in by hand and you would be amazed at how rocks, sunken logs, etc just weren't a problem. Think of it this way, if you had a hundred fish in the net they were all going different directions, so you basically provide the difference maker in getting the net in to shore. Granted, there were times where the fish would literally have me skidding into the lake as well, but that only lasted for a little while.
The real reason there's not a decent commercial carp fishery in the state is because of economics. Back in the 80s, we got 6 cents a pound for whole carp. All you had to do was pack them in 100 lb boxes with ice and ship them out to the cities of St Lous, L.A., Chicago, and NY. Other species like buffalo, catfish, goldeye, drum (any non-game species was legal) and they were gutted and sold on the half-shell (as I called it). Crappy way to make a dollar, even tougher for ugly guys who commercial fished to get a date when they smelled like a fish.
Commercial fishing in ND is a very seasonal thing, earlier than late May and starting sometime in August and the carp were out in deeper water swimming around in schools at the surface. Made them tough to catch on a regular basis. The owners of Grasteit Dakota Fisheries out of New Town went to Helena for the winter and fished there with lower overall labor and other costs.
Note, I have no idea what the price per pound is today for carp, but a lot of Grasteit's profits came from the incidental catches over the other non-game species (catfish, goldeye, drum, etc). Some of those species are now game fish and that would make them illegal to catch and sell.
FWIW, even with all the carp we took out of Audubon, I don't think anyone was ever delusional in thinking we dented the population of carp there. Audubon is the carp capital of ND, while Sak is Mecca for buffalo (damn if Beaver on Oahe doesn't hold some big buffs!
I have never asked NDGF why a person can't bait carp in with grains. It really does work!