[IANAL]

My impression is that GPL (or another copyleft style licence) would create incompatibilities with the business models of many of their customers. Taking contributions would also be problematic since Microsoft may wish to dogfood their opensource for proprietary products.

For better or worse many of their products incorporate licensed technology from other companies under terms incompatible with GPL. So GPL's strong terms don't seem a viable and reasoned alternative.

GPL might be seen as a strong option for companies whose business is built around dual licensing their technology and when all that technology was built in house or licenced under different less stringent terms.

Microsoft might not be happy with the GPL itself, but given they are working on LibGit, and not all of GitHub (or any?) of their main software they "sell" (github enterprise, their client apps like GitHub for Windows and OSX) are open source, Microsoft would probably have no issues with the GPL. It would also be interesting if MS did buy GitHub. Wasent one of the GitHub founders an employee of a company who was bought by Microsoft and he either dident move over or left after a minimum time?

I think most people have issue with GPL to be honest. So they aren't alone there.