This is another misunderstanding about hypnosis brought to us by stage hypnotism. The reasoning behind this thinking is typically this: “The hypnotist tells him to cluck like a chicken -> he does so -> he wouldn’t do that if he weren’t hypnotized -> the hypnotist controls him.” It seems logical, but it isn’t. The fact is that all hypnosis is self-hypnosis, and therefore you cannot be made to do anything in hypnosis that you couldn’t be made to do in the waking state. The hypnotic trance is a different state of mind, but it’s not a stupid state of mind; if the hypnotist crosses the line, the subject will likely wake up from the trance.
Think of it as a mental contract between the subject and the hypnotist. The issue has some expectations about hypnosis, and the hypnotists fulfill these expectations. But if the hypnotist does something that the subject does not wish to participate in, it’s the same as breaking the contract and terminating the hypnotic trance. Following this analogy: You write your contract! So if you have a belief that the hypnotist can control your mind and let the hypnotist hypnotize you anyway, it’s the same as including the following into the contract: “The hypnotist is allowed to control my mind.”
This is most likely why some people will cluck like a chicken. They believe that the hypnotist can make them do so, but he might not be able to make them give up their credit card information. A second theory is that the volunteer subjects are people who don’t mind acting a little foolish.