I’m not going to come over all Lynne Truss about this. The apostrophe is, like many punctuation marks, something that is not quite as black-and-white as people imagine. And there are worse writing crimes than missing an apostrophe.
But it is nevertheless such a basic mistake to make that you want to know how it came about. Think how many people must have looked at this advertisement. Think how much money it must have cost. How is it possible that nobody thought to double-check whether the punctuation was correct? Did people, collectively, avoid raising the question for fear they might be considered stupid or illiterate?
And, more worryingly, who was it, I wonder, who lost their job over this? The ad agency? They may have, should have, lost the account, I’d have thought. But if one person was fired, I bet it would have been the humble proofreader – unfairly, because they are not paid to overturn decisions that have been made higher up the food chain; but predictably, because they will have been deemed to have been the last person to check it before it went to press.
It’s like the goalkeeper being blamed for letting a shot in that a striker should never have been allowed to make because the defender should have got to it first and should never have been selected by the manager in the first place.