As a previous commenter pointed out "people know windows". In a previous life I worked with Point of Sales software for a multinational, and while both linux and windows systems were offered people invariable chose windows - the reasons were all different but always boiled down to the "Nobody gets fired for buying IBM" argument (or in this case Microsoft).
As for the hardware, I agree with you that the one machine per screen setup is inefficient, but while it is not the best technical decision it may be the best business decision. Almost certainly the computer running the screen is built into the screen assembly itself making a simple unit which can be sold as a scalable commodity that which can be shipped and wired up by relatively unskilled labour.
Additionally having all the machines running off a central machine would present a single point of failure - instead of one broken screen, 100 could show a BSOD on a failure or worse...
I would be surprised if there wasn't a central update facility, but that said the prospect of some nasty cracker putting something on all the TFL screens would be enough to scare most managers (who would have their jobs on the line) and enough to push a fat support contract to TFL and the advertisers which is where a lot of these guys make their money anyway.