Let's keep this simple. You must support yourself if you don't use an OS they support.
It's that simple. They have to ask because to set it up, their setup software runs on that.
Now there are some that get uppity over that question but it must be asked.
Bob
i just signed up for AT&T Elite dsl and while doing so, the service rep asked me if i was "using XP or Vista?" i actually use a Linux distro, but do have XP dual boot capability, so i said XP. when i mentioned Linux, she said something like "that's not XP or Vista is it?" she had no idea what i was talking about.
i bring this up because i thought it was none of Att's business what OS i use. i don't shop for high speed Internet service often and wonder if all ISPs ask this kind of question?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic