No harm?
Just because they can't reproduce doesnt mean they wont cause any harm.
By the sounds of it, these things demolish a big chunk of plant life in the water, which isnt a good thing... especially when half of it goes back into the water as waste.
Sure, them not being able to re-produce is great, but that doesnt have any bearing on the damage they do while they're alive.
What if a few hundred or thousand get into our lakes..... that's A LOT of plant life gone, and A LOT of waste put into the water which isnt good for our native and self-sustaining species.
I just don't get why people cant respect the rules. It's for all of our own good.