You are presuming it's a mistake, I believe it wasn't, so unless the person involved admits it nobody will ever find out.
This is true .
My only problems with incidents like these is even if it was not a mistake and the shop worker genuinely was discriminating and using his views on servicemen as a reason not to serve them . This only proves a problem with that shop worker . And he should rightly face some action for acting like that .
But whenever stuff like this happens the same old types come out of the woodwork suggesting it is a problem with all people of that race or religion which is obviously untrue and unhelpful for the tensions it goes on to create .
Yet when some white football hooligan goes home and beats his wife up because his team lost the game , the white majority in this country would all be up in arms if anybody tried to suggest all white men where like that .
For me if this is not a mistake then it is only an isolated incident that should be viewed in isolation . Its not like every newsagents run by people of Pakistani heritage up and down the country does this kind of thing .