Earlier quoted context omitted.
Fuck their shitty TOS, call the SEC and have them investigate, they need to comply with the law. They are acting as financial services provider and must be in compliance with those laws. The law gives the OP a clear resolution. There's a pretty clear resolution here, give the customer back their money because your business doesn't actually work, or give the customer the coins at the most favorable price.
BTC exchanges aren't regulated in the same way as stock exchanges. Coinbase isn't a BD AFAICT, so they don't have the same type of fiduciary responsibilities. They also aren't registered as a bank or as a NY private bank, so they aren't subject to FDIC or other banking regulations. They are much closer to ebay than to IB
That's the whole point of this, CoinBase doesn't want to be like IB so they should refund the customer his money rather than acting like a fraudulent IB.
There are plenty of more serious operations like http://bitcoincapitalpartners.com that take client money seriously.