This is another misunderstanding about hypnosis, that is 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 wasn't hypnotized -> the hypnotist controls him". It seems logic, 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 is most likely to wake up from the trance.

Think of it as a mental contract between the subject and the hypnotist. The subject has some expectations about the hypnosis, and the hypnotists fulfills these expectations. But if the hypnotist do something that the subject does not wish to participate in, it's the same as breaking the contract and thereby terminating the hypnotic trance. Following this analogy: You write your own 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 the reason why some people will cluck like a chicken. They have the belief that the hypnotist is able to 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 subjects who volunteer are people who don't mind to act a little foolish.