"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.
Which is of course not the real reason.
The reason Google doesn't follow the robots.txt protocol is (1) they don't want to (2) they can get away with it.