Why? People used to buy Half-Life just to play Counter-Strike.
Arma 2 is an amazing game but people who get it just for DayZ MOSTLY ignore it as it's a Hardcore military simulator ( more or less ). DayZ is certainly much more casual than the original game and a lot of people like it but it's still barely in Alpha and has a lot of flaws - at least in my opinion.
It looks like a waste for the people who originally got Arma 2 for the game itself - combined operations costs like 30 bucks but every single cent is worth it for an Arma 2 player (Like me for example). DayZ is still just a very new mod that is in ALPHA - on top of that it has barely anything to do with Arma 2 itself.
So some players find that a bit annoying and / or feel it's a waste of money.
I do not really blame people who buy Arma 2 CO just to play DayZ. I can understand that people just want to play the mod in peace - it's their money and not mine.
I just don't like DayZ players who call Arma 2 stuff for no reason without really trying it.
I should really stop making such long posts in this topic.