It doesn't have to be written by a lawyer. The robots.txt file is an administrative directive, by the webmaster of the website, that you, being a scraper, MUST NOT go to page x and/or y, or MUST NOT go to directory z. All the law would have to say is that it is a crime to not obey these directives. It's similar to trespassing: if I put a sign that says "DO NOT ENTER" in bright red letters on a door in my apartment, or "authorized people only!", that is still legally binding and a court isn't going to care that it wasn't lawyer-authored. The court will only care that you were told to not enter that area, but did so anyway.