They have been victims of massive USD chargebacks. I think Coinbase is bleeding so much money to chargebacks that co-founder Fred Erhsam figured out company is never going to turn a profit and resigned.
Coinbase is a Bitcoin-Fiat company, and fiat is reversible while Bitcoin is irreversible.
With easily transferrable assets it gets tough. The charge back window is on the order of 30+ days.
You buy BTC via ACH at $X and a week later it's $X/2? Chargeback "I was hacked!". You can fight it as the merchant, but it takes time, tons of documentation, and dealing with traditional bank compliance departments that a) don't know what BTC is or are trained to distrust it, b) are trained to protect their customers, c) are overworked, and d) have no vested interest in resolving it correctly irrespective of evidence.
You can get ACH charged back for up to 180 days with more involved procedure.
Chargebacks for small amounts are often paid by the bank itself because it's cheaper than actually disputing with the merchant, so banks will certainly terminate accounts of customers who abuse that.