I agree with you but I don't see the point in lending someone your head and then demanding the settings not be touched?
It's my equipment, I can demand and lay down the rules as I see fit.
Very good reason for not letting someone change the dials, on a three band bill the changeover can be tight, especially if there's a club on after the show. If you're professional about it your head will be ready to go when you hit the stage. It won't be if you've allowed the support who were supposed to bring their own in the first place fiddle with it beforehand, with a three channel head - that's a lot of fiddling.