"Your do not enter sign uses font we don't like, so we'll just ignore it"
The intent of robots.txt is to help crawlers, for example, to keep crawlers from getting stuck in a recursive loop of dynamic pages, or from crawling pages with no value. robots.txt is not for banning, restricting, or hindering crawlers.
That's not to say that I disagree. In most cases robots.txt is not legally binding. It only becomes a legal danger to not follow it when the person running the site has power and can buy a DA to indict you.