Exactly. There is some circumstantial evidence that Tether printing may be inflating bitcoin prices: http://www.tetherreport.com/

But it is just circumstantial at this point. There are a lot of ifs related to Tether, but it has that fishy feeling to it.

While I think there is a very good chance that Tether is not backed by USD, I think the regulators may actually be more interested in USDT being used for money laundering and non-reporting of investment proceeds. In fact, that is likely a larger crime with respect to US regulators.

That would be the SEC's domain. If CFTC is getting involved it's probably because they are potentially manipulating the price on a regulated futures contract.

But bitcoin futures didn't start until Dec 17, and the subpoena was sent Dec 6, so that would imply they're messing in futures based on some other commodity.

You are correct about the dates but the regulatory planning and preparation for the CFTC regarding the futures contracts has been a long process. CFTC had jurisdiction and interest in assessing asset pricing conditions leading up to the contract launch.