Its not injustice. Its business.
So, WotC is being "unfair" by not providing a Mac version, or a Linux version? It's simple business. If WotC thought they could make back their investment and earn a decent profit, they'd put out a Mac version. It's really that simple, "fair" has nothing to do with it.
There are several surveys that indicate that Mac penetration of the gaming market was higher than WotC's market research indicates, and I have enough buddies in the computer game programming biz to know what it takes to be practicable.
The simple truth is that its nearly as cheap to make a web based application platform neutral as it is to make it a hassle to less popular platforms to use. By not doing so, they've shifted an otherwise trivial cost from producer to consumer- something I reject in any form of business.
Or, to put it a different way, would you buy T-shirts from a company that charged $1.50 more per increase in size? $1? $0.50?
Would you eat at a restaurant that charged more to fat customers? To skinny customers?
Again, they have every right to use whatever business model they want (as long as its legal), and I have every right not to use it if I disagree with that model.
I got tired of my Mac-using friends making these sorts of complaints in righteous fury long before D&DI was ever a gleam in WotC's eye. Entertainment software on home computers has always been a Windows game with the Mac users losing out, it's not a new thing at all. The reality is, if you want to play games on your computer, get a PC!
My personal experience tells me otherwise. When I got my Apple 2e, there were far more games for it than for PCs. After the Mac became Apple's platform, it wasn't until maybe a decade ago that I had any problem finding high-quality Mac games, usually the same ones my PC loving buddies were playing. Even with PCs taking the lead, I could usually find Mac or platform-neutral versions of the top PC games pretty readily.
And even today, I can usually find quality games. My only problem is that, inexplicably, those retailers who stock Mac games don't stock them all...not even all of the popular ones.
To illustrate, a buddy of mine told me about a game from his company that was available in Mac, PC and Playstation format, and the versions could be networked for multiplayer play. I bought it- and it lived up to its hype. Its sequel, though, could only be found on about 1/3 of the same retailers as the first game.
Its not that there are no Mac games, its that they are not well publicized and not well distributed.
Actually, come to think of it, I wonder how many Mac-users would be willing to shell out the dough for an XBOX 360 or PS2 or Wii and yet balk at spending $400 on a mid-range PC for D&D purposes. Just random curiousity.
Personally, the last console I bought was an Atari 2600, which died in 1991. I own a Wii, but it was a gift (though I do buy games for it). I wouldn't buy a PC for D&D.