I think it may be more of a grammatical error that has been accepted into common language due to advertising. 'Brain' is singular and 'Brains' is plural, that is correct English. But when you have products describing a singular item as if it's multiple, people read it and take it as being correct, but in fact it is actually poor grammar. That jar of pickled pigs brains, should read 'pickled pig brain' if there is actually only one in the jar that is.
Poor grammar in advertising is one of the main causes of the English language becoming bastardised.