Your logic would be spot on if Microsoft's Mac offering were indeed direct ports of their Windows counterparts. But they are not.
One of the major reasons the MBU exists is because of the horribly ill-fated idea of throwing out the Mac-native Word for Mac 5.0/5.1 and using the Word for Windows 2.0/3.0 codebase to come up with Word 6.0 for both Mac and Windows. (You can read details of this debacle directly from a MBU employee).
The majority of Microsoft's Mac products are complete re-implementations of the features in their Windows counterparts (and the reason why there is not complete feature-parity between the two versions).
I think the reason that the MBU has such high profit margins is that are fewer "cooks in the kitchen" relative to Microsoft's Windows-related projects. It's a small group of dedicated employees who are in the unenviable position of trying to make great Mac software while being viewed by some on the outside as just more drones from the evil Windows empire.
To all of the righteous "downloading is a crime" types: get some priorities and complain about things that really matter, children.
Wait. What about how illegal downloading effects the children of the intellectual "property" owners (who, by the way are hardly ever the intellectual property creators). My god, at this rate they will only be able to afford 1 italian sports car and not 2 or more!
Mystics always hope that science will some day overtake them. -- Booth Tarkington