They have to spend money to check whether Linux will run on each machine, even if it turns out that they can't.
Depending on the support contract, that might or might not be a big deal. It's also possible they have a large customer who has requested that, so they already have a process.
Adding another operating system option will cost money in production, ordering etc. All to support an operating system that they then can't charge you money for, as if they charge the same price then people would likely order it with Windows just in case they ever need it and then install Linux themselves.
Who said anything about supporting an OS they can't charge money for? Of course they are going to charge money for it..
Free to them doesn't mean free to the consumer.. ;-) Of course support costs need to be built in.
Also, adding anything will cost money.
That means nothing. It's all about ROI. If it costs money, but they make more money selling it, they'll do it..
If it costs money, and they don't make money selling it, they won't.
It costs them money everytime MS releases a new version of the OS. Doesn't mean they won't support it...
If/when it appeals to customers, then they will support it, regardless of the "cost" to them. That is just the cost of doing business..