Actually, two different companies are handling Baldur's Gate and Baldur's Gate II for the Mac. Baldur's Gate was published by Graphic Simulations Corporation; they contracted Scott Warren at Rosetta to do the port. The project was massive in scope and just about ludicrous to think that the game would come out as flawless and completely without bugs. Both Rosetta and GraphSim are continuing to address the remaining issues with the game (as evidenced by the new private beta that is available today -- see games.macnn.com).
Baldur's Gate II: Shadows of Amn will be published by MacPlay and I believe they've contracted Contraband Entertainment for the port. They'll almost definitely use any code that they can from the Baldur's Gate port, but Bill Heineman at Contraband is a veteran programmer and is one of the most talented and clean coders in the biz. With the size of modern games, it's nearly impossible to ship a bug-free game but the hope with every developer and publisher is to fix as many as they can find before release and then be prepared to address those that come up once the game is in the users' hands. Even Diablo II, which went thru a rigorous beta test with hundreds of thousands of testers worldwide, came out with some bugs and the company is still addressing them.
Anyway, I think everyone who has experience in feeling burned by purchasing a game that comes with show-stopping bugs can related with you, but it doesn't hurt to look at things from the developer/publisher's perspective and cut them a little slack for the situation that they have to contend with as well.