Turbine didn't tell them to put the cards on discount in the store. I'm fairly certain Walmart itself decided to put it up on discount. See, Walmart buys the cards and Turbine gets a flat fee. They don't send out cards for free and then get payment WHENEVER one is sold.
There isn't any harm in them informing the player base of a general sale going on. Certainly it's a little silly to cry foul and complain about the lack of a sale currently.
It's like noticing you have the option to buy discount peanut butter WITH jelly added already in one jar, and then complaining the normal peanut butter isn't on sale.
The cards are an option for people who don't want or feel comfortable with obtaining TP in another way. And Turbine has so much on sale and so much on rotation for sales that there's hardly ever an absence of a deal going on. Just have a little patience and you'll get the specific deal you are yearning for eventually. Complaining about not getting special deals does not normally bring about an urge to give special deals, at least not at my DnD table.