The $15 cap was for Legend of Grimrock, which was the highest priced indie game thus far, so my original statement was erroneous and the cap is actually $20. GOG have contacted me and asked me to clarify that, which is fair enough.
There's clearly a bit of a backlash against GOG going on in this thread, and that was not my intention. It is entirely up to them what they choose to stock or do not stock. We're obviously disappointed by their decision and feel they've made the wrong call (for instance Frozen Synapse sells for $25 minimum and has shifted 300,000 copies - albeit though you get a spare key when you order it too).
However, the reason why we posted up the fact they'd turned us down was to stop people keep asking us to get our game on their service rather than to create a backlash against them. If you're keen to buy Xenonauts, every other major distributor we've spoken to is keen to stock us so you'll have ample chance to buy the game elsewhere.
@Starmaker - unsurprisingly, this is due to development reasons rather than deep-seated hatred of women.