They knowingly took a risk and did nothing to control the crowds. There has also been reports that people were congregating because they needed to be close to the shop to hear their names being called out -- that's on the operator.
IANAL, but what power does a business have over people on the public street outside their shop? Surely a shopkeeper can't force someone to do anything, except on their own property? So, between the footpath and the counter, they can enforce 2m separation. But on the public footpath all they can do is to ask, I guess? (Do we know that they didn't ask? Do we know that they did nothing to control the crowds?) And, if that doesn't work, then I guess they need to call the police. Who did call the police?
It seems to me to be grossly unfair to hold someone responsible for something over which they have no control.
That is for them to work out if they want to stay open. All the Cafes I have seen today were all complying with the rules and it would appear that KFC are doing all they can.