Say, tesco allows 50 in, it is still 50 if it is 50 singles from 50 households or 50 in couples, but only 25 households.
Having 2 infected people in one place doubles the risk of passing that on, unless they're literally only occupying the same spot and only touching the exact same things, and breathing out in the exact same space. If out of the 50 households, 10 are infected, shopping alone means 10 people breathing the virus out. If they're all Shopping with another person, that means 20 people breathing the virus out, increasing the viral load in that area.
Then there's 25 other households (plus any other households over and above that 50) waiting outside, rather than all 50 households going through.
Not all supermarkets are in out of town areas with masses of room. So there's a potential problem, plus people needing to walk past the queue, safer queuing together outside by all scientific accounts than inside, but a risk none the less, one that can be avoided.
Secondly, there's the increased waiting time for the people outside as 50 people from 25 households go through, followed by another 50 people from another 25 households and so on. It effectively would double the waiting time. Why should someone wait twice as long, because someone else wants to shop with their partner/mum/teenager (notice I said wants not needs)
Then not everyone has a car, so increased use of public transport, which has a knock on effect to those needing to use it for work etc if they can't get on because it's full. It also again doubles the risk of having two infected people in that space rather than one.