It is usually strictly regulated. In some jurisdictions you must hold it in specific accounts that have particular attributes and you must have dollar for dollar parity between the account and the money you collect. In almost every situation where you are holding money for someone else you aren’t allowed to use it to pay your own debts. And in this case the collateral obligation is on Robinhood not on their account holders.
The problem as you can see is that you deposited $100 and they have to use $15 (for example) more of their own cash. If overnight the 15% margin requirement changes to 30% and RH doesn't have a huge amount of cash sitting around they are going to get squeezed.