I talked with my Sensei aobut this, he said it probably has to do with the lack of any control over the fall in such throws, compared to the typical Aikido Ukemi in which one elects to fall to "save himself".
This 'control over the fall' is given to uke by nage. In kaitenage we don't grasp the neck, we simply weigh it down. In shihonage for people not capable or ready for the breakfall, we loosen it up. Sometimes nage forgets, especially in goofy koshis.