Comment by esoterica

It's not about the relationship between the end user and the broker, it's about the relationship between the broker and the clearing house. Trades take two days to settle, and the brokerage needs to post collateral with the clearing house to reassure them that they have enough funds to settle the trades. When volatility goes up, clearing houses demand more collateral to allow further trading, which the brokerage might be unwilling or unable to provide. This is independent of whether the end user is using margin.