This may be a daft suggestion, but perhaps they were black pacu? It seems unlikely that anyone with such a desirable fish as piranha would give any to Walmart, considering the ridiculously low prices Walmart pays for their fish. For the record, I believe juvie red-bellied piranha have spots like red-bellied pacu do.
Yes, juvenile red belly pacus and juvenile red belly piranhas both have spots, but both outgrown their juvenile markings and colorations into slightly altered forms. If indeed there were piranhas in the pacu tank I wouldnt doubt it. Aside from the mouth there is no real way to distinguish the two species. An experienced dealer would clearly know the difference, but we're talking about Wal-Mart so who knows where their shipments arrive from.