A lot of it is ignorance. Many people assume that any disappearance in a tank with a mantis is the mantis' fault, when more often than not, there's another cause. *cough-cough poor husbandry habits-cough-cough*, it's just easier to blame the mantis.
That said, they do eat clean up critters. I had a mantis in my tank for over a year, and I lost fish - to a sally lightfoot and an emerald crab.