I was trying to figure out a situation where this could possibly matter, and it was difficult. Even considering top-decking, it's not possible to play treasure cards after you start buying (and you can't Embargo the Black Market deck, right?), and the top 5 cards in your deck end up in your next hand regardless of order. Nonetheless I was able to find a situation where it not only has a game impact, but isn't entirely stupid. If you buy from a supply pile with 10 Embargo tokens on it, with three Hagglers in play, and a Watchtower in hand, and Chapel is in the supply, then you would want to top-deck cards in a specific order so that you can have 1 Chapel in each of your next 3 hands. If all Embargo tokens are a single event, then it is not possible to top-deck all 10 curses and have Chapel show up in your second hand.
When a player buys a card, he gains a Curse card per Embargo token on that pile.
To me, this wording implies a singular event, because the trigger only occurs once. I.e. on every buy this event is triggered, which hands out a variable number of curses. Note that, under this reading, the event is triggered for non-Embargoed piles as well, it just hands out 0 tokens. If the token count were involved in the trigger instead of the event, there would be a stronger case for multiple events.