Oh, yah... there's no question that people are buying them. I suppose the "lots of metal in the building" argument can be made. But if you get caught with one on the wall behind the insulation in your attic, you'll get a fine from Uncle Fed.
The only argument I've seen that seemed legit, other than the whining from the cell phone industry and consumer groups like doctors & lawyers, is the possibility that the jamming signal could carry outside the boundaries of your private establishment and effect people walking (or driving) by.