That's a really good point, and you're right, there must be something to it besides just 'thinking.' Actually, now that I'm thinking about it (heh), the same logic applies to thought-speak. Why can't an infested person use thought-speak, if all you'd have to do is think at people to have them hear you?
With thought-speak, I've often considered the fact that the Animorphs never 'accidentally' thought-speak (except Jake, one time, can't remember which book), so it must be something more than just 'think at person x' because that would happen all the time by accident (I know I don't have that much control over my thoughts, at least). Therefore, I believe that, to thought-speak, you have to actually activate the speech centers of your brain, but somehow without physically speaking. This, a Yeerk would be able to block.
Morphing must be similar. There must be some region of the brain that controls it, to where it becomes more like flexing a muscle than simply 'thinking.' Therefore, a Yeerk can prevent it from happening.
P.S. And, yeah, those Auxiliaries who managed to 'accidentally' morph in #50 were probably a KASU. Or at least had neurological conditions that gave them less control over their actions than the average person (Tourette's, perhaps?)