I think it's a perfectly legitimate way to have a sale. Think of Amazon's daily deals. There are pros and cons of each technique. NOE may be thinking that dribbling out the sales will make the choices seem less overwhelming and make a habit out of checking out the eShop. I don't see how buying eShop cards can be an issue. I can either add value directly using a credit card, or buy a digital eShop card and redeem the code immediately. It's all pretty instant.
This is where I've been buying my digital eShop cards: https://www.paypal-gifts.com/us/brands/nintendo-egift-card.html