Despite whats been banded about - this wasn't necessarily intentional... the branch of the engine that the game is developed on does not have any native AA path at all and relies on it being enabled through the control panel on any hardware... the developer included a vendor specific custom multisampling path for nVidia hardware but disables it on other vendors hardware citing lack of testing... now why they didn't include an ATI specific path or a generic DX AA path is open to speculation... or why someone didn't take the time to properly test the custom multisampling path on other hardware.
The official line is they asked ATI to contribute to a path for ATI hardware or testing the custom AA implementation and ATI had made no real progress by the time the game shipped.