I'm kinda stumped about this myself, as I recall AMD made the claim that Freesync range is dependent on the panel's capability of variable refresh. Now here's this thing: TFTCentral pointed out that the panel in this Asus is the exact same one found in the Acer XB270HU, and that monitor is capable of variable refresh rates all the way up to 144Hz so the issue of the Asus not being able to do 144Hz freesync clearly has nothing to do with the inability of the panel to do so itself. It looks like Freesync, at the moment, is simply not capable of variable refresh rates up to 144Hz on IPS panels?? There are currently no other Freesync 144Hz IPS monitors out there atm so it's hard to say but I'm pretty glad I decided to stick with NVidia this time around and get a G Sync monitor.
Its not the panel it's the scaler that the manufacturer decides to use. Freesync works with whatever range the scaler can support.