I disagree. In my opinion most people use it on retainers because they're metal and JB Weld is an high temp epoxy system designed for bonding metal. I've stopped using JB Weld for affixing motor retainers and instead now use a standard structural epoxy and they don't fall off mid flight/post motor burn. NFPA 1125 states motor casings can't exceed 225 deg C (+/- 5 C), which a good epoxy system can easily withstand.
Mach 2+ flights incur heating (primarily) on the nose cone and the leading and trailing edges of the fins. In comparison, the fin fillet does not see anywhere near the aerodynamic heating effects. And once again, if motor casing heating was an issue for a composite airframe it would soften the airframe/motor mount first, not the fillets on the other side (and therefore insulated from) the motor casing. Also, most people that I know that fly MD and are looking to hit extreme speeds don't use a motor retainer full stop as the retainer 'step' induces unwanted drag. Better to use an internal motor retention system that doesn't induce drag.
Because it's not necessary or fit for purpose. It has a specific use case, bonding metal; beyond that there are better systems out there (ProLine 5400 and EpiGlue come to mind) for fin fillets.