It's difficult to know the precise reason behind them accepting a 2.2. They might want to consider a broader range of candidates so that they can (e.g.) accept someone with a strong background in business or entrepreneurship who simply didn't achieve a 2.1 in their first degree. I suspect that the ability to attract sufficient applicants to fill the places is another driving force.
I'd be surprised if most employers knew the admission requirements of individual MSc courses. I suspect that the Warwick brand would carry across but, of course, you couldn't say you studied at "Warwick Business School" (only "University of Warwick") if that's what you wanted to do. I'm not in the business/entrepreneurship field but would be surprised if this distinction mattered very much by itself.