The IRS started doing something.
https://techcrunch.com/2017/11/29/coinbase-internal-revenue-service-taxation/
I think that's a part of why Bitcoin has risen so fast recently...people don't want to take profits and pay taxes. That and that there's no practical way to short it until the futures arrive which I think might be tomorrow or soon.
Interactive brokers already said cannot short CME and CBOE BTC