I always thought they were illegal because of the danger to the local eco-system. Much like the snakeheads...so please correct me if I am wrong. You see I can have a Red Asian Aro or a Snakehead here in Canada, but if I release it in a river/lake it won't make it through the winter so no danger here we know it will die. But, over in the US of A, where there are more temperate weathers and even close-to-tropical systems, they would thrive in your rivers, and endanger a lot of native species.
You can thank a few imbeciles who released foreign species in the US for that...