From what I can gather the barrier to green card sponsorship is the immense cost. So OSM literally charge you all the legal costs (taking a cut themselves) and get you employment.
I have actually emailed OSM about the scheme and that was what I got from them. Even though you pay all the costs they seem to think you owe them for the chance. They bond you to an airline for 4 years and if you leave before that kiss your money and green card goodbye.