Alltogether I have to admit, I don't see why this is such an issue here? I know stuff like this simply as one aspect of what is taught by a whole lot of teachers.
Is this really so unusual for some of you - even for those whoe are practicing for such a long time ... - or am I getting something wrong?
As far as uke is concerned: There are certain teachers and schools who teach ukemi by bridging. Over here this is especially popular within the Tendôryű. And there are simply some aikidôka who like to do ukemi this way. I myself don't think this to be a clever position. So I think, it is not something tori does, but a certain way this uke reacts. No miracle here also.
As I have said a couple of times in this thread, I think this kind of demo is about showing something abstract. A technique executed at speed may look like a very linear application of force, while the teacher is actually doing something subtle, so they slow everything down and expand it out so you can see that it's a rather more helical movement. Unless the teacher explicitly states that they are showing an application that should work on an unwilling partner, I assume it is not a demonstration of an application.
And the students, I think, cluelessly believe the teacher is demonstrating a skill, or something that can be "made to happen" without the cooperation of an uke. So then further down the line they naturally believe their teacher can actually do these things, while other teachers making the same type of demo are fake. Perhaps the teachers do or don't help this delusional process along by sucking up the admiration or just refusing to actually tell the students that.
Another problem is that there is a matter of degrees here. What is the difference between allowing your instructor to bend you all the way backwards when you could probably try to roll or turn away, and kicking your feet up into the air so you go flying at the end of a kote-gaeshi or irimi-nage, when you might be able to do something else that would allow you to keep your posture? Why hold on to a wrist that you know is going to perform a technique?