Hmmm . . . I haven't attended any of those yet.
I did become interested in bonsai by a demo, but it was done by a local club, presented by one of their own with mostly their own in attendance. The presenter made a point of demonstrating various techniques, using nursery stock but not trying to turn it into a "finished" bonsai, merely showing how to get started with it and using some of the club's more developed trees, which were on display, to point out various aspects of the art. It was very well done, I thought, and obviously it was interesting because, well, here I am today, learning the art myself.
I don't know, but I would suppose most of the demos you are talking about are not given for bonsai clubs, but for garden clubs or other groups of that nature. They are probably not attended by people actively learning bonsai techniques, but rather by people who enjoy the "finished" product. Maybe a few care to learn about bonsai techniques, but I'd guess most will find it interesting but not necessarily something they want to do themselves. To that extent, it may be relatively harmless to do an "instant bonsai" demo for them.
On the other hand, the demonstrator could certainly talk about the differences between what he or she is doing to the nursery stock and what is actually done in bonsai development. They could have a couple of more mature examples on display to point out features of a developing or "finished" bonsai. It would be very simple to work good information into such a presentation, as well as information on caring for and further developing the "instant bonsai" that is destined to be raffled off to someone who probably won't know what to do with it.
Of course, a lot depends on the exact circumstances, but presenters worth their salt know that they need to tailor their presentations to their audience. What one does in front of a garden club and what one does in front of a bonsai club would not be the same. Also, what one does to entertain a garden club would not necessarily be what one does at an arboretum plant sale where one hopes to draw in and hook potential new members for the local bonsai club, even though the kinds of people one gets there may be very similar to the kinds of people attending the garden club demo.
In these matters, one size never fits all.