I had no idea it was fairly easy to copy. Though checking Wikipedia, it looks like there's no easy way to get the images.
That said, there are lots of other things that I'd generally consider sufficient justification, but some of them (like e-mail and chat) can just be installed on the LAN, and others (like Stack Overflow and Github) are a bit computing-specific (and why computing professionals need Internet access is more of a no-brainer).
More importantly, if you don't have Internet access, you won't discover that something like Wikipedia might be helpful for your job.
Maybe I just don't have experience working in non-creative office jobs, and there really are people who never have questions that can be best answered by the Internet.
Distraction is a Symptom of a Deeper Problem: The Convenience Principle and the Destruction of American Productivity is a good article on distractions versus getting things done. With extra emphasis on how many of our distractions are the result of a desire for convenience rather than something more substantial.