robots.txt isn't a standard. It is a suggestion, and not legally binding AFAIK. In US law at least a bot scraping a site doesn't involve a human being and therefore the TOS do not constitute a contract. According to the Robotstxt organization itself: “There is no law stating that /robots.txt must be obeyed, nor does it constitute a binding contract between site owner and user, but having a /robots.txt can be relevant in legal cases.”

The last part basically means the robots.txt file can be circumstantial evidence of intent, but there needs to be other factors at the heart of the case.