YABU.
I work in a boarding school which has stayed open for a minority of boarders throughout the pandemic. The vast majority went home to their families, but a few had to stay.
This is exceptional as it usually closes for holidays, but we understand that:
(a) some children can't go home! (Yes, the UK is letting them leave. But their home country might not be letting them enter. Or there may be no flights, or the quarantine rules in their countries are very strict. So just saying that the UK is allowing children to go home to their parents is only half the story.)
(b) some could go home, but would not be allowed back in the country again. We have a number of students who stayed as their country had been added to the Red List, and their parents preferred them to remain at school with their teachers and friends than be forced to do government hotel quarantine alone at a young age.
(c) some children live in different time zones. Our teachers have been running a full set of live online lessons - it's not really fair to expect a student whose parents live and work in Hong Kong to be up at 3am to attend a Drama class.
As for enjoying the facilities and having a marvellous time... well, it wasn't great fun for them when they all had to isolate in their solo dorms for 23 hours a day when they first arrived, so I'm not sure you why you don't want them going swimming in the school pool or doing a bit of exercise outdoors. They were not, of course, allowed off site.
OP it seems like your real issue is with sending students abroad for school in the first place, not with them staying open for a small number of international students who can't go home during the pandemic. What would you have forces/diplomatic families do then - move their children to a new school every year? And do you really begrudge people spending their own money on educating their children, if they can afford it?