The key is that neither game has been refused space on the platform -- Microsoft has simply decided not to publish the games itself. Microsoft requires all XBLA titles be attached with a publishing partner, meaning neither Amanita Design and Golgoth Studio can self-publish the games on the XBLA platform. Sony and Nintendo do not have the same policy for the PlayStation Network and WiiWare, respectively.
Joystiq contacted Amanita Design founder Jakub Dvorský for to clarify the situation. "They told us, 'It's not Microsoft-exclusive, we don't want it.' They didn't cite the Mac and Linux versions [as reasons for the refusal] but it's quite clear that's the reason," Dvorský told Joystiq via email. As it did with Toki HD, Microsoft recommended Amanita Design pursue other publishers, an idea the team doesn't appear intent on exploring. "It means if we want to release the game on XBLA we must throw, probably, most of the profit out of the window -- to the publisher. Just because we created Mac and Linux versions."
Microsoft's official stance -- and policy, according to Dvorský -- is the company does not publish games that are not exclusive to its platforms. During the year-long process of waiting for a response from Microsoft, Amanita Design never expressed interest in submitting Machinarium to Sony's PlayStation Network. Now that Microsoft has decided against publishing the game, Amanita is exploring Sony's platform. "We submitted the game to Sony after Microsoft refused it," Dvorský said.