It's common practice in classified environments, as a necessary security precaution. But it's a true productivity killer imho. Better to just install an extension that blocks twitter/facebook etc, than completely cripple your ability to search for technical information as you need it.
Also - good luck installing tools and plugins when you have to start copying things from the internet computer to the classified computer.
With godoc, man pages, info pages, a local copy of the Common Lisp Hyperspec and Usenet, what more does one need?
If you're worried about using code from stack-overflow, use it as inspiration only and write your own code. Or conduct a short search first to see if it WAS copied from another code repository.