I believe the reason it won't default to Bluetooth Audio is because it's not linked right at start-up. In the few seconds it takes to sync to your Bluetooth device, the radio detects that there is no connection and switches to another source. I have to same problem with music on a USB flash drive. If the radio doesn't connect and read the media files quick enough, it'll switch to FM radio. All this happens within a second or two during start-up.
2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts