Why do you care how much money the other person gets?
As stated, you're never going to see them again, so the only thing that matters is how much money YOU walk away with.
The decision is a psychological one, not a simple, probability-based one that you suggest.
If everybody just did "steal", then no-one would ever win anything. Eventually people would realise that to win something, there needs to be some sharing going on. That's the interesting bit: if you both realise that some sharing needs to happen, can you genuinely convince the opponent that you ARE going to be sharing (and then screw them over....).
If you played this over and over, people who always "steal" because they don't want the other person to get anything or they're scared of looking like a gullible tw*nt on TV will always end up worse off than someone with a varied approach.
This is the level at which the game plays. The players are already past the "well, I might as well just steal" stage that you are talking about......
Still a rubbish program tho'!
Of course, the best approach is to tell the opponent, "I'm just going to steal, but I'll give you half the money afterwards" (this deal cannot be performed in the "Prisoner's Dilemma" because there the reward can't be split).
An irrational person who just gets annoyed that you said such a thing will steal and end up with nothing. A rational person will realise that they might as well trust you, seeing as they'll be no worse off anyhow.
In game theory, the situation you describe could be defined as a "Nash Equilibrium", which is a bad situation for a game, because it means everyone should always steal, as you describe. However, this is only bad in a game if the only thing that matters is the result relative to the opponent. In a game show, like Goldenballs, the only thing that matters to the players (or the only thing that should matter to rational players) is their own absolute result relative to their start position.