> If the story about Robinhood running out of money is true, why wouldn't they just ban buying on margin and not all buying? I don't understand that.

Because DTCC requires cash collateral while the transaction is settled whether the transaction happens in a margin account or a cash account. There's no distinction from DTCC's perspective. DTCC (reportedly) increased the collateral on $GME transactions to 100% of the transaction price compared to the typical 1-3% of the price.