Oxford colleges have had to make hard calls, because the pressure on our support staff and the local community is already at problematic levels. For example, there have been complaints to the police, including submissions of CCTV footage, of groups of undergraduates congregating on the street over the past few days, blocking pavements for residents.
Where students are returning because they genuinely could not access their course at home, we expected they would continue to follow the spirit and letter of stay at home (in their room within their college) once they got here. Certainly our plan at my college was that once we saw things were safe, and our staff could manage, we would endeavour to let more return. Unfortunately after just a few days that has not been our experience.
We have needed the wisdom of Solomon, and I'm sure we've made some misjudgements - allowing some of the sharp elbowed back who could have accessed their course at some level from home, but got requests in early (optimal access of the course is of course as a resident for all of them, but impossible) and sadly missed others who need it more. Unfortunately we had to close our appeal process once we'd heard from our junior deans and then the police about problems even already in 0th week. And at this point, to be frank, we're anticipating needing to send some home if the bad behaviour continues.
Hope this helps to explain the situation a little more. Anyone worried about a student who is unsafe or cannot access their course at all from vac homes should tell them to contact their welfare advisor. Perhaps those who have returned will settle down when work begins and we can fit in a few more, but the more those here ignore stay at home, putting our support staff and neighbours at risk, the harder that is.