I don't know, but I doubt that's the reason. Trading/selling still involves gralats, and gralats mean players still buy gralat packs -- Unixmad still makes money. In fact if players were to sell hats they'd probably be selling rare hats and rare hats mean they're not not being circulated anymore: Unixmad wouldn't be making money off of these... but if there were trading these hats would probably sell MUCH higher than shop prices on hats, so chances are it'd be done by someone buying lots of gralat packs.

Anyways, I'm not in a position to say what their reason is.