Both GTA V and IV and Liberty City stories can be downloaded and played on a Mac but to do this you have to get a hacked emulated game or whatever. This says to me it is possible to play on a Mac so I don't know why it can't be developed and sold. The current speil from Rockstar is they don't want to release the game until gameplay is up to speed without bugs. In the mean time people are downloading it anyway through the hackers. Why not produce what they have as a beta and make it downloadable since most new Macs don't have Disc drives anyway. People understand if they have a beta version it will have issues and I don't know of a game that never had issues, in fact sometimes it makes a fun part of a game. Hands up those who would be willing to pay for a Beta version just to get something decent from Rockstar instead of the Hackers on their Mac.
P.S. Watch Dogs is coming out soon so if it's on mac then GTA V may be too late and Rockstar will miss out on a fortune.