They want you there to answer questions.
Workplaces value availability because it's too difficult to design coherent systems and to document them. Instead of that boring old stuff, in the modern workplace, we go fast and break things (or never get them fully working in the first place).
Then we expect workers to be always around so they can be reached on slack when there is a problem.
But hey, I like surfing, and you can't 'reach' a person while they are surfing. ¯\_(ツ)_/¯
I don't like it either, and I feel like I could contribute a lot more effectively on the fixed price items if I wasn't constantly scrounging up T&M work, maybe I'd even have time to think up decent solutions instead of justifying why I have to spend another 2 hours on issue #blah, but it's an example of why required hours exist.
I got sidetracked.. what I mean is that you need HN, Facebook etc to prevent you from using too much of that precious brain power. Maybe this is different for other areas of work, but IMO it should make not much of a difference whether you are forced to stay at work for 35 or 45 hours - you just will procrastinate more.
Update: Added “more than”.
But use it, and it will tire. Forget to pause and it will tire faster. And neurotransmitters are not an endless resource.