What a rubbish reply! It is NOT an incorrect use of GRAMMAR it is an incorrect use of punctuation! First they claim it is a mis-spelling which is meant to be 'quirky' (how exactly?) then they claim it is grammar. i think they're making it up as they go along!
Companies have a responsibility to get spelling, punctuation and grammar right!
Re the Apostrophe Protection Society - they go a bit over the top sometimes. They issue 'improvement notices' to errant breachers of the apostrophe!