Wyoming is not noted for it's granola population, but it's not legal here to keep game fish without permits, nor is it legal to transport any live game fish without a permit. This practice is based on fishery management more than anything else. People don't realize how big most game fish will get in an aquarium (no predators, regular feedings and higher temps, usually), and then don't know what to do with the huge fish they have. Very few LFS will accept them as turn ins, so people dump them in a local waterway, with no thought to the consequences.
Illegally 'stocked' fish have ruined many native fisheries, as well as spread/introduced diseases. It may seem like a joke, but think how you'd feel if you came home and discovered an oscar had been dumped in your tetra tank. Same thing, only many times a breeding population is established. In a lake, one bass might seem insignificant, but a breeding population will decimate a trout fishery. Not a pretty picture.