Manipulating customer margin calls becomes trivial if you can look at the books and calculate if buying/selling x amount of the order book will margin call y amount of funds and decide its worth it.

>Coinbase, Inc. is licensed by the Virginia State Corporation Commission as a money transmitter, but such license does not cover the transmission of virtual currency

On top of that, we're talking about the big players like Bitfinex, who created USDT simply so they didn't have to comply with any sort of banking laws, let alone securities trading regulations.