Why do people do this? Wonder if they ever catch any of these people who illegally release these animals?

When biologists say the pacu fish eats nuts, they may be correct in more ways than one.
The pacu, a toothy fish that can weigh up to 55 pounds, has been spotted in Lake Lou Yaeger in Illinois, KSDK reports.
Responding to a report that a fisherman had reeled in a piranha on June 7, lake superintendent Jim Caldwell brought the fish to the Illinois Department of Natural Resources, where it was identified as a pacu. Some reports say another pacu was seen a couple of weeks later.

http://www.huffingtonpost.com/2012/0...n_1656015.html