If there are Windows users that already have them then chances are there are Linux users that already have them (for their other [other ...] OS partition).
Exactly. I'm a Linux user, I already have a copy of Torchlight (which, incidentally, was a fantastic game and well worth the price on its own). It's sold well over a million copies by now, there is going to be some overlap in that number and Linux users.
If there are Windows users that already have them then chances are there are Linux users that already have them (for their other [other ...] OS partition).