I totally have to agree Martin, nothing is free in this world. I havent looked too much at Robinhood but there is no doubt you are right. Someone always cover the cost, and it is for sure not the owners of the business.
I have seen recently an increase in forex trading scams again, some of these are for free as well (in the past they asked for some kind of subscription). The 'professional' manager will trade your funds for free with a shitty broker, what people dont know is that most retails clients are on a B book, no real trading ever take place - shitty brokers will then spilt the money they steel from their clients with the so called professional asset managers. Their goal is never to make money from trading... Sadly most of these scams go mainly after poor people in Eastern Europe and Asia, with a promise of thousands of percent return and no risk.