That can't possibly be true. I know some Walmarts have rung up those games before their release date.
A few years ago, one of my local Walmarts (I lived in Chattanooga at the time, but I forget if it was the Walmart on Gunbarrel Rd or the Walmart over by Lil Debbie Parkway) put out the new version of The Show days before the release date.
(In Walmart's defense, that was the year when The Show made the questionable decision of having Joe Mauer repeat as the cover athlete, so the cover looked vaguely the same as the previous year's game with it being the same guy with his same swing in front of a blue-ish background -- so I'm not surprised it wound up on the shelf early since it was probably mistaken for the previous year's game by a careless employee.)
Anyway... the only reason I didn't buy a copy was because the PS3 version had
already sold out. They only had a copy or two of the PS2 version still left on the shelf.
So they had obviously been ringing them up and selling them.
I wouldn't be surprised if PS4 versions wind up on shelves early this year too (depending on how lazy/careless your local stores' electronics department staff is). It'll be too confusing for some of those workers to understand that they're not supposed to put the PS4 version out when they've had the PS3 and Vita versions on their shelves for weeks.
It's just luck-of-the-draw whether you have a store staffed with people who care (or who know what they're doing) or not.