Off-road vehicles aren't held to the same emissions requirements as road cars, so it would be grossly unfair to remove catless systems from the market entirely while there are still legitimate applications for them. Parts manufacturers and retailers have no control over whether their systems are installed legally, so a lot of the burden rests on shops who touch the emissions systems or re-certify vehicles for road use. Tampering or defeating emissions systems remains federally illegal in all US states, but not all states have regular inspections, so everyone has to decide for themselves whether or not they're willing to risk getting caught. The risk is higher for shops, who can be fined several thousand dollars per violation. At any rate, retailers are more than happy to turn a profit selling these products, as all they have to do is warn the customer that the product is "for off-road use only". The rest is out of their hands, and it's up to the customer/installer to ensure that the parts only end up on vehicles that are only operated on race tracks, farms, backyards, etc.
1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver