I'm pretty sure a friend's app got a bad rating when he declined to sign up for a service like this.

How did that go down? You would think it would be easier to just not talk to someone who solicits this.

My understanding is that he got dozens of offers to help promote his app. Some were blatant buy-a-download - just to gain app store rank - some were less blatant, others were more traditional buy-a-targeted-clickthrough. It can be difficult to tell at first glance.