It’s ok to post guards at showers for security.
If requiring ID when inside its also ok to post guards outside to restrict incoming traffic.
It would be wrong to prevent people from leaving. That removes the equation they are there by choice.
I work at a facility that has armed guards at entrance. I have to abide by their rules but I am free to come and go as I please . So nothing wrong with my rules.
I used to work detainee-ops overseas, so I'm familiar with guards at the entrance. It may be alarmist/overblown but the article says "under lockdown" ,which implies to me that they are not free to leave.