No doubt they were aware of Mac user's frustration with IE, and thought (correctly, I would think) that there was a market for a better browser on the Mac. In fact, considering the nature of the Windows browser market, a Mac edition could have doubled their share over a Windows version alone.
Unfortunately, they weren't the only ones. That's business, that's life. Whining won't change a thing, and certainly won't make you any friends.
You do have to wonder what, exactly, they though was going to come of a hissy-fit. Maybe they should threaten Microsoft too; if they're so sure it will have the desired result, then it follows that it will double their chances of killing a competing browser. Still 2x zero is still zero.
The real issue here is Apple's marketing clout, which is formidable. Every "iApp" and high-end commercial program Apple is promoting existed as another product in the marketplace before it came in-house. If Opera Mac had been develped even a little earlier, chances are Steve would have cut them a big cheque instead.
