The FAA seems to have a flexible definition of "business purpose". They allow a non-commercial rated pilot to take potential customers on demo flight if the pilot is the salesman. That is a clearly commercial or "business purpose", yet they do not require a commercial license to do it. But if you want to charge someone $25 to take them in the same plane on a 20 minute sight seeing tour, then you must have a commercial rating. Same aircraft, same flight, different ratings required.

In both cases the pilot is being compensated and earning taxable income as a result of the flight.