Second, search engines don't always respect robots.txt. They sometimes do. Even Google itself says it may still contact a page that has disallowed it. [0]
Third, robots.txt is just a convention. There's no reason to assume it has any binding authority. Users should be able to access public HTTP resources with any non-disruptive HTTP client, regardless of the end server's opinion.
[0] "You should not use robots.txt as a means to hide your web pages from Google Search results. This is because other pages might point to your page, and your page could get indexed that way, avoiding the robots.txt file." / http://archive.is/A5zh8