Fraud is never morally permissible.
If they peddle it to the dying, it is fraud. Suppose that someone has an idea for a space propulsion system, which may or may not work, and raises money to build it. If it does not work, according to you, he is guilty of fraud. Those who sell cryonics are making no exaggerated claims. They clearly state that it is a gamble, and leave it up to the individual to decide if he or she is willing to take the gamble.