Some of restriction laws are a pain, however because attention was not given or thought about we have problems that will never be reversed. Carps, kudzu, pythons, cobras, crates and too many to list loose in the U.S. are problems.
Look at Florida, especially around the airport, monkeys, parrots, snakes, iguanas and everything else that got loose is turning the everglades into a safari. Native wildlife can't compete with much of this. Heck, most people can't deal with the problem wildlife either.
If you get a Wels catfish and put it in your lake, you are not problem free. With a catfish that can live 30-100 plus years, it can out live you. Then what happens to the catfish? Chances are your family will not want to continue with your pet or care to make sure it can't escape.