The best way I've found to explain this is to emphasize that to a recruiter, I am the product. The recruiter is trying to get me a job, just like a car salesman is trying to get a car a driver. Of course, there are good recruiters (and good car salesmen) that are truly interested in finding the best match for both parties, but at the core of the relationship the employer is the one paying the finder's fee.

That being said, the best recruiters take the long view, and realize that their career depends on maintaining relationships and growing their reputation. If they become known as someone who simply puts butts in chairs, they won't get very far. However, if they develop a reputation as someone who really is looking out for everyone's interests, their words will carry more weight with their customers, and they'll have the best product to sell.