Who's "they"? My local PetCo sells them. I read up on them very briefly. They are filter feeders that will eat whatever little microorganisms they can filter out of the water.
I had one for a week... very cool little guys... mine was damaged by the petshop employee trying to catch it, and then lost most of it's front legs moulting... and died a few days later.
they filter algae and food out of the water column with their fan like hands. they like fast currents and are hard to feed if you have crystal clear water.
Oh they are awesome, I have one guy in my 55(I think I forgot to include him in my sig!). He chills under the outtake of my XP3 hidden around plants. They filter feed as said before, extracting plankton and other microorganisms out of the water...hence it is important if you want to keep one to have an established aquarium with a good area of flow. They do not need any other food, if one is seen scavaging the bottom of the gravel in search of food...it is a bad sign their eating is not being satisfyed.