That device costs thousands of dollars. The os license fee is around 35-50. It barely even registers in the grand total. Add to that the well known MS management and reporting tools and everybody choses the MS variant. They know it, they can use it and the store staff can even do first line support themselves lowering the tco even more.

That's it.

Edit:typos

I think it's just because that's what most people know as an OS and know how to use it. Although every company in the world would save money by using Linux for these things since it is free.