IE does not exactly follow standard HTML specs, so they are using markup that is IE specific, therefore their site only looks 100% proper on IE.
MS has been doing this forever, it's nothing new. There was a big controversy awhile ago because MS allegedly made the msn.com site not display properly on Opera. They were using a browser detection script and loading a different CSS for opera than they were for other browsers (this was long before the firefox explosion). After Opera countered by changing the way their browser was detected, MS altered the site again to make it not work. This went on for about 2 weeks until MS finally relented. It was a whole bunch of underhanded BS from MS.