Maybe that Mac user wouldn't purchase quite as much Mac software as another one, but it's still a Mac, and it still sells some Mac software. The Internet's all about open standards , being platform agnostic, if you will. Apple's in a perfect position to capitalize on that trend. Not only are Macs equal to PCs for most Internet tasks, they can, if needed, turn themselves into a PC via a software transformation. In the past companies just whipped out code any which way, using whatever development software the lead programmer prefered.
Programming is big business.
Code has to be written so it's easy to port. That's a slight bit of extra effort in planning, but it pays big dividends when it's time to release multiple versions of your product. For instance, RealVideo wouldn't be the standard it is without Mac support. This is only going to become more pronounced. The future is all cross-platform.