There are a couple of reasons that you should be paid your regular rate for sleeping. First, you're not at home, in your own bed. You must maintain a constant state of readiness, be ready to respond within one-two minutes. Even if you're on call from home when off duty, there should be compensation in the form of a stipend. In either case, you're not free to go about your life, you're being compensated to be available for calls.
Second, the company saves money by having you work 24 hour shifts. This allows them to cover more shifts/hours with less employees. Think about it. For every five employees on a variation of a 24/48 no kellys (56 hour workweek, on average), you save 80 hours of labor a week, the same as two FT employees. That's a savings of 2 retirement plans, 2 medical plans, sick days x 2, paid time off x 2, equipment, uniforms, increased support/admin staff, insurance, a savings in hiring and training costs, so on and so forth.
If you're working, you need to be compensated at your full rate for the entire time, not just if you get a call. That reduced "sleep rate" is a load of bull. If you get no calls at night, the company makes out since they're not paying you to be there, even though you're still covering their rig, making it available. If you get a call, you get your full rate only for the time worked. Sounds a little one sided, no?
Why stop at non payment for sleep hours? What about all downtime? I was watching a news artcle from Canada that brought to light this very practice in some rural areas. You had these medics working a massive amount of hours as paid on call from the station, in hopes of going FT. "Must have been in BC"
If you don't think employers are doing whatever they can to shortchange their employees, google "chinese overtime" and "system status management".
Edit: Bottom line, if you're not at home, in your own bed, you're working. If you're expected to staff a rig for a call, you're working. If you're working, then your full hourly rate applies.