I was an independent contract instructor and spent many a day at the airport for free when I first started--trying to get students. The difference was it was voluntary. Also, as an independent contractor, I charged whatever rate I wanted (I always charged for ground and flight time) and kept every penny. Once I established a client base, I relied on my reputation and didn't need to do it anymore.

I made a pretty good living this way, actually a little too good as it made me reluctant to move on and take a pay cut. 16 years later (after 12 in the AF) I'm making less my first year at US/AA than I made per year as a CFI.