Public aquariums end up with them due to private hobbiests who swear up and down they'll upgrade before the pacu gets too big, or just don't have a clue. I have yet to see an aquarium BUY a pacu.
We don't like them, but people always threaten to just dump them outside when they call in and find out we don't want them. You'd be shocked at how many people that they are doing the aquarium a favor by "donating" a fish or some other aquatic pet.