I've been to two hackathon type events. I would have gone to more, but I was living in an area without much of a startup/hacker scene.
The first was SuperHappyDevHouse, following Startup School in 2006. It was mostly hackers working on their own projects and occasionally collaborating. I saw no evidence of anybody attempting to exploit anybody else, especially not non-hackers trying to get free work from hackers.
This was the second: http://hackd.it/. I've gone a couple times now, and didn't feel like there was any exploitation going on there. The ratio of hackers to non-hackers was good and people seemed to mostly be working on things they had a stake in.
A similar event run by non-technical people looking to get their ideas built cheap/free wouldn't be enjoyable for hackers, I expect. If you know an event is like that, don't go.