...if they were so profitable, wouldn't the company trying to get you into it would set it up themselves rather than getting other people to?
I remember Don Lapre's infomercials in the nineties advertising some kind of home-based business, and from what I could tell at least one part of it was a rip-off. There was one thing he tried to get people into that was like this: he set up a bunch of 1-900 numbers, and all you had to do was market them and you'd get money if people called them. I'm assuming he got a cut of it too. Well, 1-900 numbers probably aren't expensive to set up, but the risk you assume when trying to run them is that the cost to advertise it to get people to call might be more than the money you make from the calls. So basically he was getting other people to assume the risky part of setting up these 1-900 numbers.
More on him:
http://en.wikipedia.org/wiki/Don_LapreThere's lots of consumer watch information on the web about more specific ventures too.