Robinhood markets themselves that they are taking from the rich and giving to the poor. Their behaviour is the exact opposite of that! They're actually more like loan sharks or payday loans.

Transparency is concerning (as are all pricing strategies that try to hide the ball), but in the world of finance this doesn't rank very high in the opaqueness department. As you say, if you're not the customer you're the product. Especially in the context of a service that is actually performing real-world, meaningful transactions on your behalf (as opposed to publishing funny comments to internet memes), you have to be hopelessly naive to believe you're not remunerating them with your own money.

There are plenty of hopelessly naive people out there, but you don't slap down a business strategy simply because it could steal from these people. You wait around until its clear the harms outweigh the benefits, otherwise the market in every product or service would be limited to minimizing the potential harm to the dumbest among us.