According to a possibly groundbreaking judgement by the U.S. Ninth Circuit of Appeals, it is allowed to extract data from publicly available sources.
It is via the use of the CFAA that US law enforcement may categorise some types of computer activity as "hacking".
Because they were harvesting LinkedIn user profiles, the biggest professional social network said they were breaking their terms of service and were committing hacking.
However, after losing the first litigation of the year, LinkedIn still hasn't given up on winning the second one.
On LinkedIn, our users have entrusted us with their personal information, which is why we restrict scraping on our site."