In comparing a Kyokushin roundhouse kick and a MT kick the defensive mechanism for the kicks is much different, and that is due mostly in part to upper body positioning when the kick makes contact. In Kyokushin the round kick (assuming mid-high kick) keeps the kickers posture more squared at the shoulders, they bring up the knee and turn the hip over snapping the kick out from the knee and "into" the target, driving their weight or sinking their weight into the target as kick makes contact. This creates a more squared up stance at the shoulders into impact and brings the upper body forward..... given the mechanics of this kick, it would be stupid to drop your hands because the upper body positioning would leave you easy to counter.
The difference in a MT kick is that there is no (or very little) chambering and the kick drives "through" the target vs. "into" the target. The point of contact in a MT kick is after the upper body moves through (or across) the target, instead of sinking the weight into the target (bringing the upper body forward) the MT kick transfers the weight all the way through the target which distances the kicker outside of the opponents striking range (if they throw the kick correctly). So they hand or arm throw is done to get the hips all the way through the target, by the time your hand is thrown back on the kicking arm your upper body SHOULD be through the target and outside of striking range of opponents hands, while the other arm forearm guards your face to avoid counters as you retract the kick.
In short, the arm or throwing back of the hand can be both poor technique or proper technique depending on how and how well the kick is thrown. Both ways effective, it is just personal preference. I prefer a chambered round kick to set up hands behind (and yes, i keep my hands up) and the opposite applies for a thai kick as i prefer to set up the kick with my hands. For all intensive purposes i use the Thai roundkick much more often because the damage created is much greater whether or not the kick is blocked and attempting to counter a MT kick with your hands is a brave move if the kicker is really throwing the kick with bad intentions, to counter you have to step into that kick....... Why throwing a slap ass or sloppy MT roundhouse is a recipe for getting KTFO (Maia vs Marquart), but same could be said for any kick you throw half ass with lousy technique.