I guess if the non-vulnerable person has symptoms & tests positive and they move out they are more likely to spread the lurgy to another household, so perhaps that's why they have to stay put?
But if the vulnerable person has been in the same household as the infected person they could theoretically be brewing it even if asymptomatic, so if they move out surely they could potentially infect the other household?
Although if you are shielding exactly as they prescribe you should not even be making eye contact with each other, so they are probably assuming you won't catch it from your ill partner. 
If you're vulnerable & move in with another family does that make you immune to catching it off the other family, then?
Problem solved! Let's just all move in with other people!
DS has just returned from school. Yr 12 but the first time he's been in since lockdown. Probably the last time before September, too. They had a mock exam.
I asked what school was like, was it very different because of social distancing etc? Apparently, apart from bigger spaces between the exam desks the only difference was some trees had been cut down outside school & one of his mates now has a car. (visions of The Inbetweeners).