If it is classified as a threat to local ecosystems, then I can see why it would be illegal.
There are many cases of plants and animals being brought from other countries or areas and introduced into new environments where their natural predators are no longer there to control their populations.
http://www.thebutlerchain.com/introduction.htm
"The Butler Chain of Lakes has had some problems with exotic, noxious aquatic weeds that were brought in by boats. Aquatic weed control is an expensive, not always successful, program."