MS is now a services business, they want to sell you their cloud. They can't do that if the linux ecosystem is shut out of Windows as it was before. Simply supporting Linux VMs is not enough, because long-term they still want to maintain some control on the OS market. So they open enough doors that you (or your corporate boss) will be enticed to consider their offerings and move to their cloud. You have some sweet cloud-management tool that runs on Linux? "You can now run it in the same way on Windows! Oh and with Windows in our cloud you also get all these other goodies..." - et voilà, locked back in.

Most "advanced" cloud services are built with the aim of locking you in (AWS, Oracle, Heroku, etc etc): easy to adopt and easy to extend, which means "hard to leave". Microsoft is no exception.