The public good
The Linux Foundation has already let the fox in the henhouse. That was the actual terrible thing that happened. Since that's a done deal, though, I think that letting them onto the linux-distros list is not only reasonable, but desirable in terms of the greater good.
The list is primarily focused on security issues, and whether we like it or not (I obviously don't) Microsoft is doing Linuxy things that are impacted by, and will impact, Linux security. They need to be an active participant on that front.