My point is this would be an unfair clause, no private business would sign terms that would force them to work for free. Yet, somehow it's enforced and acceptable for private individuals.
In our case it's not enforced, and we're not under any 5 nines obligation.
On the other hand, we're a bunch of lazy sysadmins and don't prefer to work if it's not necessary. As a result, we design our systems for resilience, so any problem both doesn't get big, and can wait the next morning, or next Monday 99% of the time.
Also, we definitely love what we do, hence we sometimes choose to take some work off of our colleagues' hand on a slow night. So, I personally prefer to read a tech doc, learn something and apply to on our systems if I have nothing better to do instead of watching TV.
IOW, we're not losing sleep doing our job.
TL;DR: It's acceptability depends on the circumstances a lot. If the employer is abusing you with that stick, that's bad. Otherwise, I see no problem.