I've been told that the idea behind the law was (at least among other things) a matter of fair competition. That if a shop started discounting like crazy on just one item, folks would get inside for it and eventually would be shopping there only. And most times, the bigger store has an easier making those discounts because they have a safer profit margin where the loss will largely be compensated by the variety of items a single customer can buy in a single visit. Now it doesn't mean that there isn't other mecanics that could invalidate this reasoning, but it's the main argum that was appare put forward in the discussions.