Calm down people, don't accuse this guy of ripping people off (other than Air Miles maybe) and doing illegal things without knowing what Air Miles even is. Air Miles is a Canadian retail loyalty programme found at many service stations, supermarkets, etc. The cashier did everything as normal, but when he asked the customer if he had a loyalty card and they didn't, he probably swiped his own. Air Miles is independent from all of these companies so the clerks who scan the cards at retail places have no need to promote them. The customer paid for their own fuel, he simply collected the "unused" miles, which as said typically accrue at a paltry 1 mile per $20 spent. (However, short-haul redemptions start at around 1k miles) Air Miles is not going to be happy about this, but he didn't hurt the customer or his employer in any way. Actually, this is quite ingenious... this guy must get quite a few free flights a year on this, particularly if there's a 10x miles promo or something going on.

This isn't Aeroplan or credit card related at all.
Yes, you can use an Aeroplan card at many retail places to earn miles in Canada as well (usually 1 mile per $ spent). This is actually quite disliked by people who actually fly AC for their miles.