Because people want to play the game, but they don't want intrusive security measures. Yes, you can say "Well fuck that, I'm not getting the game because that there is some stupid DRM", but some people actually want to play the game despite this.
I find it funny that you posted that demotivator on bad analogies, when both your WoW-analogy and your PC requirements analogy aren't too good to begin with. WoW is a multiplayer game where as AC2 is a singleplayer game through and through. The game itself gets zero benefit from being online.
And compairing it to not having a strong enough computer, is like Ubisoft saying that you need a Radeon HD5970 to run the game, and nothing less will do, when in truth, the game will run well enough on a Nvidia 8800GT. Except here, there are actually benefits (better graphics).