Why not? Alot of times in the tech industry especially, prices are jacked up crazy, then discounted to give the customer the illusion that they are getting a good deal. It pays to do your homework before buying anything.
The thing is, these top up cards already have a price stated on them, so the false discount is very obvious (unlike the tech industry example you mentioned). So how come the sellers still get away with the misrepresentation?
It's like Acer engraving on its laptop that it costs $2000, then Courts stating the retail price is $2200, then selling it at a "special discounted price" of $2000. If Acer already stated the laptop retail price is $2000, you would wonder, shouldn't the discount of $200 make the price $1800?
Of course Acer or any computer manufacturer will not engrave the retail price on their products, but SingTel prints the retail price in BIG BOLD numbers on the top up cards and makes it a public record, so how can the seller jack up $2 more above it and then give a $2 discount and still get away with this cheat?