That's according to a Japanese retailer. In an interview with Japanese mag Game Lab, the manager of Tokyo retail shop Games Maya dishes about how downloadable content is faring in brick-and-mortar stores.
For pre-paid PlayStation Network Cards, the manager says, "We don't see a single profit." The reason? The PSN uses an actual monetary unit, so according to this retailer, "That card becomes a voucher." Thus, the store sells the PSN Cards at the marked retail price point.
Conversely, since both Microsoft and Nintendo do not use monetary values, these pre-paid cards do not count as vouchers. Thus, the story is able to buy and sell them at different price points.
Since we don't work in retail, we can't confirm any of this — any further from readers working at game shops would be, as always, appreciated.