But I think it is completely wrong to compare them to a library setting, or an office, when the occupant can move around and go to the toilet for more than 15 minutes a day and can lean their head on a desk or get up and move and don't need to keep looking ahead.
Well, if you're working in a call centre you can't get up for a wander. And your toilet breaks are timed and monitored. So I would say that its a fair comparison.
But, as we are talking about school children:
If you're a kid in a normal class you can't sit with your head on the desk. Because then you wouldnt be working / paying attention.
If you're a school kid in a normal class you cant get up and walk about the classroom because you feel like moving around for a bit. Because then you wouldnt be working and you would be disrupting others.
If you are in isolation (at any school I've taught in) you've got work on the desk in front of you that needs completing, so you need to be looking at that.
Really, there aren't massive differences between the expected behaviour in the classroom and the expected behaviour in isolation. But because those students in isolation have - usually - not met those expectations the rules are spelled out more clearly and the physical environment is designed to facilitate getting kids to follow those rules / meet those expectations.