"It costs a licensing fee. It has more security liability than pretty much any other choice."
Yes, it does when it's hooked up to internet with no protection. That isn't this case. I may be entirely wrong, but isn't it the case that an unconnected (except for a highly secure private network), fully patched Windows XP machine is no easier to break into that an equivalent Linux / OSX machine.
"Linux costs nothing to license. BSD costs nothing to license. Windows costs something. That's an added, unneeded cost."
The licensing fee means you can blame them when it's their fault. If you want to blame someone else with Linux for a fundamental OS security issue, you'll still need to license it for a cost. That's why Red Hat make money.
"Because there aren't lots of dev tools for Linux that run on a normal desktop computer?"
Original question was wrong. Who cares, as long as the development tool does the job effectively.
"How is it easier to develop an ATM on Windows than on Linux? They both have tons of tools and myriad experienced developers and companies. Linux is probably better optimized for appliance uses and has a larger share of the appliance market than Windows, making it easier to find companies to work on it."
Because they've been doing it for years so it's far easier to port from old Windows to new Windows rather than rebuild the whole things from scratch. There may well be a new, better technology, but it's a hell of a lot cheaper to regression test against a newer version of an existing platform than it is to rebuild for an entirely new one.