So customer A is doing stuff where you need to put up a collateral with some counterparty.
You can't use customer B's money (this is a key assumption that might be missing - it's all about the use of other customers money) for that collateral because, well, that collateral might not get returned in certain cases - that's kind of the point of having a collateral. You'd lose that collateral if the counterparty goes belly up (insolvency, fraud, whatever), and, most importantly, you'd lose that collateral if you become insolvent. That's not OK - this is regulated so that you are required to ensure separation of "your money" from "customers money that you're holding on their behalf", so that the customer's money is untouched and unclaimed even you go bankrupt. It's not your money, it's the customer's money that you're investing on their behalf, so you can't put it up as repayment or collateral for your liabilities; and you can't put one customer's money as repayment or collateral for another customer's liabilities.
But why can't customer B's money be used as collateral for customer B's trades? I guess because the collateral is all comingled when it is supplied to the clearinghouse?
You can't use customer B's money (this is a key assumption that might be missing - it's all about the use of other customers money) for that collateral because, well, that collateral might not get returned in certain cases - that's kind of the point of having a collateral. You'd lose that collateral if the counterparty goes belly up (insolvency, fraud, whatever), and, most importantly, you'd lose that collateral if you become insolvent. That's not OK - this is regulated so that you are required to ensure separation of "your money" from "customers money that you're holding on their behalf", so that the customer's money is untouched and unclaimed even you go bankrupt. It's not your money, it's the customer's money that you're investing on their behalf, so you can't put it up as repayment or collateral for your liabilities; and you can't put one customer's money as repayment or collateral for another customer's liabilities.