This is quite wrong; it's still considered a crime of stealing from the bank. The law does not care if you used a computer to do it instead of using dynamite to break into the safe, nor should it.
"Another way to put this is that you are using the contract in a manner how it was defined by the author."
No. If you are using an undisclosed exploit, then that is not true in the least. If you are using something that is not equally known to both parties, and equally disclosed, then it is fraud. No amount of, "you should have known better" or "you should have done your research" will help you. Take the recent case of the hack involving Etherium. It exploited something in the "smart contracts" which very few people knew existed, and, judging by the actions of the team in charge, clearly was not meant to be there. But beyond that, if the contract were to be disputed in court, one thing that would become very clear is that no rational person would be expected to believe that was the intention of both signing parties. And thus, it would be decided that the contract was fraudulent.
"If everything is decentralized and open to anyone, whose computer network are you intruding/disturbing?"
So if I forget to lock my door, then my house should be free game for everyone?