The guideline limit is not targeted at those who have a little stash at home (sensibly - in my view - I have some paracetmol, ibuprofen, loperamide and a few other bits and pieces stashed in the bathroom cabinet since you never know when you may need it) - but those who may be in some distress and about to do something foolish. It does not take much effort, if determined, to buy excess - you could go back around and go to a different cashier, or go to another shop down the street - but supposedly the research shows that this inconvenience is sufficient to put people off.