Occasional browsing is one thing, but when half of one's work day is related to doing nothing, then it becomes a problem.
I was responsible for a tri-branch office managing the network for about 800 desktops, 50 servers, and 300 employees on the clock around the clock. Nobody wanted to do what I did, so I had to do it by myself. Management wanted bandwidth reports. I gave it to them. Wanted email stats. Gave that to them. Over half the email was FW: FW: FW: FW: garbage, which I was instructed to
Needless to say, people would do nothing but talk crap about me because I'm "the web nazi" or "internet nazi" and other such crap.
And having worked at such a company for over a decade, one day, out of the blue, the same management that told me what to block was the same management that said after XX years, if I didn't take a 75% paycut, that I could take a hike.
I believe in working. If you're not working, you do not deserve to be paid.
And to this day, I still get daily status reports of all of the servers I once ran, nearly 3 years after the fact - because the guy they hired to change my passwords did only that, without checking on anything else.
I've never worked in IT since. There is no reward.