This also reminds me of an episode from the initial run of Who Wants to Be a Millionaire.
The show in the U.S. began while I was in my second year of law school, and my wife and I watched pretty regularly. The show had questions of increasing difficulty and increasing dollar value. Once you correctly answered the $1,000 question, you were guaranteed to take home at least that much money. Same with the $32k question.
One day, we were watching, and a fellow classmate of mine turned up on the show. He did really well and got to the $32k level pretty easily. He correctly answered the $64k and $125k questions correctly, but he had burned through all three of his lifelines. They revealed the $250k question, and he clearly didn't know the answer. The smart move would have been to walk away with the $125k, but he blindly guessed and got it wrong. His stated rationale was that wasn't losing anything, but that just demonstrated why he wasn't at the top of the class. He had $125k in his pocket, so while he still got $32k, he pissed away $93k on a 1 in 4 guess.
This choice is basically the same. Yes, in walking away with nothing, you're no worse off than you were before, but once you've been presented with this choice, you have a guaranteed $1M, so you're risking that money on a 50/50 chance.