I can't speak to the legality under Canadian law, but I'm certain it's against the employment conditions.

A friend who works at a grocery chain tells me that using his personal loyalty card for customers is forbidden. First, the whole point of the loyalty card is the data mining aspect, which is frustrated if all the transactions are funneled into one card. Second, they're running a promotion where you can get 10c/gallon discount on gasoline for every $100 spend. For someone working a register, it would be very easy to max this out at $1/gallon off.

They had at one time used a store card to avoid arguments with customers over getting discounted prices, but were told to stop (again, the data mining).

He's not allowed to benefit in any personal way from transactions he processes.