Maybe part of the problem is the 'work for free'. It may suggest to people if you see no value in what you can offer, what value would you be to the company.
I disagree. In the startup community, you basically work for free until funding comes through. I think this guy knew the situation and approached it properly. Offering work is one of the best ways to get hired. Offering to work for free sets the bar lower so its much easier to exceed expectations and get a job after the pro bono project.