There are three main reasons that people will attempt to contribute to an information service like Wikipedia or Yahoo! Answers.
First, there are people who contribute because they genuinely want to provide information. This is the altruism motive: make the world a slightly better place, and maybe have some fun at the same time.
Next, there are the people who balance things. If they want an answer, they contribute one back. They may not really enjoy the process, but the give-and-take, often enforced by a point system, helps make sure people don't free-ride.
Finally, there are the people that expect to get something from it. Sure, they'll contribute an answer, but they expect a return on it. Often enough, the reward is pushing a point of view, though it could also be money, advertising, or the chance to knock something you don't like.
Personally, I think it's straightforward to see that the Wikipedia-haters here are here for the third motive: they want to influence public opinion on Wikipedia, so they come to Yahoo! Answers to do that. It's a powerful motivator, surely more powerful than altruism or fairmindedness.
Of course, it might not be as strong as you might think. Gregory Kohs earlier admitted privately to me that he had used multiple accounts to cheat on Yahoo! Answers, and there's nothing stopping this from happening again, assuming it isn't already. Isn't it deliciously ironic how someone who accuses others of questionable ethics has no qualms in his own actions?