We are sorry for the inconvenience that this has caused, but it took a great deal of effort and resources to port DEFCON to Mac. We had in-house programmers working very hard (With those mentioned above) for rather substantial amount of time to bring the product to the Mac. We at Ambrosia wish we could offer you a code that could work on both Mac and PC, but unfortunately that option is not available as Ambrosia is dealing specifically with the Mac side of DEFCON, and the PC codes are purchased directly through Introversion's site. This has been the business arrangement with all the other products Ambrosia has marketing and published for Introversion. Introversion sold the PC version of Uplink and Darwinia, which codes did not work in the Mac versions of these games, nor did the Mac codes work for the PC version. It is just more inconvenient, and noticeable, with this game as it has inter-compatible network play that bring the worlds together.
Again, we are sorry for any inconvenience that you have encountered.
I'll try to address your questions specifically.
a.) the Windows serial key can be used with the Linux version,
This was something that Introversion decided, that we have no control over.
b.) the PC and Mac versions are inter-compatible for network play,
Which is why the Mac Version took so long to be created, there was substantial amount of work put into it by Mike Blaguszewski and others here to get it all worked out. That directly links into the difference in price between them both, which in turn also is a reason why there is a need for distinctive codes between the Mac and PC users.
c.) the game uses online authentication to check that a serial key isn't in use on more than one machine at a time (and therefore there's no reason you couldn't ensure that a Windows key isn't being used on two machines at once).
I don't understand it. Like I said above, it feels like an unfair and artificial restriction to make me buy two copies of the game.
Officially Ambrosia asks all it's users to purchase the exact number of copies of the products for how many computers it will be installed on. That said, our main focus was getting the game to the Mac market and having it work flawlessly with the PC users. We weren't focusing on the scenarios where in people would be purchasing a Mac copy and a PC version, considering the PC market was the focus of Introversion. So, we assumed that if the person was using two machines (of any operating system) that the user would purchase the equivalent number of copies.