That's a good point... Zimbardo could claim that anything but the agreed-upon safe words were to be understood as part of the act they were hired for. Then, I imagine that it would be very hard to show beyond reasonable doubt intent to actually falsely imprison someone.

However, the original article claimed that the consent form had no such agreement -- Zimbardo merely claimed so in a phone interview.

Furthermore, given that Zimbardo's rebuttal is 18 pages long, you'd think that he'd at least briefly comment on the accusation of potentially criminal behavior.