When you say "open fold", do you mean that Antonio wanted to fold his cards face-up, or just merely fold before acting on the last street?
In any case, I'm pretty sure that accepting Antonio's fold when fold is not an option, and having both players turn up their cards is not an explicit WSOP rule, but is done for transparency. If the fold was allowed and the hand simply moved on, viewers would be wondering what the heck just happened there, isn't that soft play, etc...
The only potentially related WSOP rule/policy that might be considered is that at showdown, if a player wins the pot because the opponent decides to fold (e.g. it goes check-check and 1st player mucks before other player shows, or it goes bet-call and the bettor folds before caller shows), the remaining person must still show his complete hand in order to win the pot. In contrast, in some other non-WSOP tourneys, you may not need to show one or both cards to win the pot so long as you are the "last man standing" at showdown.
Perhaps Jack is effectively treating this situation as check-check-muck, thus Einhorn at least would need to show anyways. And showing Antonio's hand would temper any accusations of soft play. ET is right that if the action actually went check-bet-muck, we may not have been able to see Einhorn's hand, but if Antonio is willing to surrender the pot and to have his own hand shown, then perhaps that would be considered a sufficient penalty to pay to see the other person's hand (although I don't think Antonio had any expectation to see Einhorn's hand when folding in this particular case).