I don't agree that the child should've been taken out of the shop.
I get that it's annoying/ irritating to listen to a child having a tantrum, but I don't agree with the whole ''it's common decency'' thing 
If it were in a hospital ward, or at church, then yes, common courtesy would apply, as these are places where quiet is expected, but a shop at Xmas time ? No.
Children will have meltdowns anywhere, for any reason. That doesn't mean that the parents must put complete strangers discomfort ahead of their parenting decisions
, particularly in a place where peace and quiet are most definitely NOT a priority.
We also don't know that the 'couple' were actually a 'couple', do we ? So we cannot know whether one of them was even able to take the child elsewhere, do we ? Could be the man needed to choose the shopping for himself, and his sister was paying for it......................(or lots of other possible permutations)
There's going to be a LOT more crying/ screaming kids in the shops before Xmas, so maybe try a little 'goodwill' instead of bashing total strangers in the net................?