Comment by justlurkin12

That's my question as well. I think it's because the customer capital has to be transferred to the counterparty that issued the sell. I imagine that it makes things complicated if the DTCC has to send over 1-10% of this amount and then the brokerage that executed the buy has to send over the rest so, if I had to guess, they did this for simplicity's sake. Btw the requirement to not use customer collateral is enforced by the DTCC and not at a brokerage level discretion.