The CDC warning just came out yesterday I believe (2/24). It may take a day or two to update their policy.

It wouldn't (shouldn't!) take an outright ban for airlines to offer refunds/rebookings. United is offering a waiver for Hong Kong and they're only at Level 1 - practice usual precautions. (Japan was just raised to Level 2 - practice enhanced precautions - and there's not a United waiver for there.)