Okay, a short education on piranha.
I wouldn't worry about these people introducing then into the wild, no money in that. It's the people that buy them you have to worry about. For instance when we moved from NH to SC we had to get rid of them because they're illegal there. So we just sold then back to the pet store where we bought them. In most cases that's what people would do, but there are plenty of bone heads out there that might try to flush or dump them.
Now they might screw up the Eco system, but I wouldn't worry about them killing swimmers. They might get bit, maybe but.....here's the thing about piranha, it's not always like animal planet where they strip an animal in like 5 minutes. My exhusband used to reach his arms into our tank ALL THE TIME, never bit, let alone have all the flesh from arm stripped. They were well fed.
What happens is, they live in the amazon. There's rainy and dry seasons, during the rainy season the river bloats, and during the dry season it shrinks, and pools of water can get separated from the main river. Then what happens is eventually the piranha in it eat everything that's stuck there with then and they start to starve. So if an animal comes along to drink from the water, they kill it. But that is not the behavior of piranha that has abundant food sources.
End piranha education for the day.