Microsoft releases Microsoft Linux with the kernal extensions necessary to load WinNT programs on top of this version of Linux. Microsoft does not distribute the original binary kernal, but instead make you aquire one yourself. By not distributing it they therefore do not fall under the GPL. Nonetheless, many open-source advocates chortle loudly, and demand that Microsoft ship them the source code of the kernal patch. Microsoft happily complies by sending them the original, pristine source code of the original Linux kernal they started with, state they never modified the kernal and therefore the patches, since they are not modifications of the kernal itself, but independent additions, do not have to be distributed because the patches are not covered under the GPL, but are covered under whatever license Microsoft sees fit.