That doesn't mean it's like that in most states. Some states have stricter laws regarding keeping wildcaught natives (I know in MN it's forbidden without specific permit).
To determine it's illegal in the majority of states you would have had to review the laws for all 50 states and memorize them, and I find that quite unlikely.
MANY states allow people to keep wildcaught natives for aquarium use as long as the people catch those fish in accordance with local fishing regulations, and the species are not endagered or threatened.