As you know, in life and in flight deals, there is always a catch. You can only purchase tickets a minimum of 14 days prior to your actual journey, and you won't be able book any travel from December 21-23, December 26-27, December 29-30, and January 1-2, 2019. There are also a number of location-specific blackout dates listed on the airline's website.
That said, these are pretty minimal restrictions when it comes down to it. Sure, tax isn't included in these prices, but if you can hop on a plane to San Francisco for less than 50 bucks, you really can't complain.