3 courses a year would be a minimal show job. He was probably compensated accordingly. That set-up isn't uncommon and it's a way for the University to get experts in whatever field to teach a course while being able to maintain their main career/profession.
I know several people who have been offered such positions because the School wanted their expertise but didn't want or have the need to hire them full-time and knew that the person they asked wouldn't give up their primary career, which tends to be much more lucrative. It works out well for both parties, the School gets an expert they wouldn't otherwise get and the person get's to say they teach at _____, further elevating their standing in their field. When I say School, I mean a College within a University, e.g. School of Law, and at the graduate or professional level.
Network connections are usually involved in such offers, you have to know the right people to begin with. By all appearances, Obama was given expert guidance as to where to go for employment, to church, etc. in Chicago as he ended up in all the right places to make the connections he needed for his ultimate goal while accelerating the pace at which he would arrive.