I have to ask again, why are machines that are not even desktop computers running windows?
People tend to forget that Microsoft was founded as a systems software company. For a long time, that's all they did. Then, later came office apps, Internet Explorer, Hotmail, etc.
As for airport/train station displays, I think early displays were LED matrices running on custom hardware + OS. I suppose just hooking up an LCD flatscreen to an embedded windows computer displaying a webpage allowed for easier maintenance and richer content.
You can either pay for a Windows license, or you can hire someone to write an OS from scratch. (You can also use a different OS, but then - again - you have to pay for someone to support and maintain it.)
You don't avoid the cost you thought you saved on in the long run. I am not making the point that anyone should write their own OS, that would be worse from a security perspective. I think though that there is a market for a stripped down OS that has less moving parts.
That's not something you'd need a Linux hacker for - just configure the distro _once_, all the remaining work is webdesign.