You don't understand how trade settlement works under the hood. When trades occur, the money doesn't change hands immediately, only on the trade settlement date

You don't understand how trade settlement works under the hood. When trades occur, the money doesn't change hands immediately, only on the trade settlement date a couple days later. Before the trades settle the clearing house needs to be reassured that the brokerages actually have the money to settle the trades (brokerages have gone bankrupt before), so they make the brokerages post collateral. When volatility goes up the clearing houses demand more collateral to deal with the increased risk, and if Robinhood doesn't want to pay it then the clearing houses won't let them trade. In other words, even if the end user isn't using margin, there is "margin" happening in the background between the clearing house and the brokerage.