GameStop is sad when it comes to stocking video games, I guess they rely on used sales for everything, but this isn't unusual as this has been going on for years at GS. Aka only stocking enough games for preorders and then never getting other new copies in, while expecting customers to buy the used one, if there is one. But for a gaming specialty retailer I expect more. Meanwhile you can go to Target or BB and find plenty of copies of whatever game you want on the shelves. My target had one copy today, I expect it will be gone soon.
Normally I'd agree with you, but I think, at least in this one specific instance, Nintendo is the one that made a mistake.
From what I've heard, Gamestop pretty much blew through their allotment from Nintendo on preorders alone. Based on the numbers other stores are getting, there is pretty much zero chance they could have gotten more this week.
It sounds like Nintendo seriously underestimated demand, and if the game is still out of stock 2-3 weeks from now, that will pretty much confirm it.