They don't make a Linux version, so by missing out Linux as an option, they'll have no idea how many already have, and how many plan to move over to it.
Linux can no longer be avoided by the publishers, and while I don't recommend Ubuntu, it has at least helped drive people to Linux.

Non-free Commercial operating systems are doomed - it's an evolutionary guarantee.
It's as obvious as the sun will rise tomorrow.

Yeah, well Linux has been around for quite some time and I haven't seen any mass exodus from Mac or Windows to embrace it.