How is getting real amateur men to fuck the idols any more crazy than all the other crazy things going on the JAV industry? What are we afraid of? VD, sure hospital check isn't 100% foolproof but pretty good. Violent or crazy fans, well if you are getting real unknowns on the set, you are taking some risk already. Having the fans/amateurs standing around, maybe with the penis out, maybe getting sucked or jerked by the idol, well that's pretty intimate/close encounter already. Sure, penis penetrating vagina is one big extra step, but I refuse to believe that it's an absolute red line that every single fan appreciation or magic mirror bus vid is total fake.
One big reason I believe at least some of them are real non-regulars is that they truly seem timid about the whole setup. A lot of these men, even after the idol has already begun kissing or sucking his cock, even after the idol has herself stripped off her top and pulled down her bra, is still too timid to touch the idol (the tits, the pussy, fingering her pussy etc), and their erection can be super-lame even by Japanese standard. Why? The fan is lacking confidence and experience. Instead of spotting a huge aching erection and trying to hold back premature ejaculation, his mind is actually more occupied by the 4 off-camera gorillas waiting to bash his nose in and rip his nuts clean off if the idol so much as make a frown.