For instance, Sandeman's runs free walking tours all over Europe. One or two every day of the year, good local guides, and the only payment is tips at the end. I'm sure the tip money is good since they get good crowds, but that seems like it would support one or two guides at a living wage, not a seemingly professional multi-national organization.

Does anyone have the inside scoop on these operators? Where are they making their money?

(I'm asking because I've found it's always good to know what is motivating people who are giving away things for free in order to avoid getting scammed.)