"Variable Wage" is exactly why I quit Advantage several years ago.  They had an in-store rate, a lower admin rate, and an even lower drive time rate.  I applied for merch work with WIS International a few months ago and they had a two tier system also.  One rate for in-store and another lower amount for admin and drive time.
I guess CAST is looking at it from another angle.  If they can pay you more than base for certain jobs such as Toys R Us, they can pay lower than base for others.  As long it it is above the state minimum wage I assume it is legal.  Anybody have factual information to the contrary?
Not saying it is right or ethical, but I assume it is legal.