Do criminals follow laws? Seriously, it is a bit of 'good faith' that bots follow the rules.
...your server makes sure they are complying to your rules[?]
Well, unless you are running your own server, no. When a robot shows they are not obeying the rules set in the robots.txt, or not even reading it, there are several courses of action we as webmasters can take, which is the topic of many a thread in the 'Webmaster General', 'Apache', 'Logging and Tracking' and 'Search Engine Identification' forums.