they are legal in about half the states, california is one of them where they are illegal but there are plenty of states that allow them... there was a town near where I grew up that had a small man made pond in the middle of a park, they had to stop renting the boats to people because there were piranah attacks

They closed the pond for about 6 months while they fished them all out... that is pretty much why they are illegal here, idiots release them into the wild instead of being responsible.
Adam