After many, many years of practice, I have concluded that beginner ukes don't pirouette 'out of a shiho-age' because it's any kind of natural response, but rather that they see the technique demonstrated as having some turning motion, and they feel awkward just standing there ... they just start turning in any direction.
My recommendation ... do not base a technique solely on how well you can do it on someone who is sincerely trying to do the right thing, but just doesn't know what that thing is..
Then my question is: why would anyone remain standing there? It doesn't make any sense that uke stops attacking after the first blow. AFAIK, the natural reaction to any kind of wrist or elbow lock (which shiho nage is during its initial stages) is to either resist or to get out. I'm either positioning myself to roll out of a throw safely, or positioning myself to be able to reverse the roles if nage's technique is unable to keep me unbalanced throughout its course.
I've tried shiho nage on people who've never seen it before and they too pirouetted out of it if I didn't apply it correctly (and they generally endangered themselves when I did do it well). It's the same thing with ikkyo: apply it to someone who hasn't experienced or seen it before and they will turn their backs towards you in an attempt to lessen the lock and/or pain if you don't immediately take them to the ground. Same case with sankyo as well. I'd even go as far as saying that a lot of more experienced people, including yudansha, tend to turn their back on nage in case of ikkyo. It's really a perfectly natural way of reacting when your elbow is being bent like that.
This is why training with absolute beginners is so enlightening: they aren't conditioned yet. They react as their body tells them to, ie. less pain = good, even if that ends up putting them in more vulnerable positions.
Then again, teaching people to be good ukes is way more difficult than teaching them to be good nages. I find that being a good uke takes a lot more concentration and physical fitness than being a good nage. You have to be able to react very
quickly from time to time.