Yeah, it is. Robots.txt is just a way of politely asking people not to crawl your site. It doesn't have anything to do with the law.
If a web server, on other other hand, willingly serves content to both a browser being operated by a human, as well as screen-scraping software, then it shouldn't try to prescribe how the screen-scraper uses that information.
It would be the equivalent to, every time, asking somebody that works in the museum if you can take a picture, and them saying "yes", and then wanting to complain (or sue) afterwards.
That's not what was happening here. LinkedIn's server was blocking HiQ, and HiQ sued LinkedIn to prevent them from doing that.