In addition tokryptonaut's comment which I think is accurate reading about their platform:
To be fair there are existing trading platforms that do this, robinhood being a prime example. However in robinhood's case I heard that they make money by charging interest on the remainder amounts left in people's accounts in USD after they use $1000 to buy something like $974.54 in an ETF. So the remaining $25.46 might be small, but in aggregate over all of their users the interest on that amount helps them turn a tidy profit.
Do you know you can get paid for your steemit blogs in which you review Cobinhood? https://steemit.com/cobinhood/@starkerz/is-cobinhood-undervalued-get-paid-to-review-oracle-d-first-challenge