I don't think the guys who pirate these games should be able to play them either, but I don't think it's a direct correlation to sales as the developer or even many in this thread suggest. At a company I used to work for, we had a pirated copy of Q3A in the sense we had 1 copy being used by 6 people. Even if we couldn't have hacked it, we never would have bought 6 copies. Matter of fact, unless I checked to see if the game was in fact pirateable in that sense, we wouldn't even have bought one.
I think that a large portion of the people who pirate games aren't going to buy them anyway. But that still doesn't mean it's not wrong and they shouldn't be able to enjoy the game without paying for it. Especially when I enjoy the game and gladly pay for it.