If it is actually true that ASDA/Walmart actively supports this "Help for Heroes" campaign, it's perfectly understandable that some of their employees might get pissed off at being forced to be tools for militarist propaganda. It isn't a legitimate part of their job.

Their job is to sell goods to the public on behalf of the organisation that pays them, whatever those goods might be, so long as they are legal. That's like saying that an anti smoker should not be forced to sell cigarettes!