I believe it is because people with weak immune systems will be avoiding these places for leisure and entertainment but they cannot avoid going to shops and even getting public transport. Or you may need the tube to get to work but have someone vulnerable living with you.
But going to pubs and restaurants we can pick up the virus and many of us will have now symptoms for days or at all. So we have to be careful around others- the elderly, and anyone with weakened immune systems.
I agree this situation will be around for a while and will likely get worse over autumn/ winter but it's the only way to protect lives until whatever scientific solutions will be available. It's worth being civic minded.