My guess are these are the remnants of store returns. When book stores order too much stock they send the unsold copies back to the publisher but because sending entire books would be prohibitively expensive they only send the covers and discard the rest of the book. I'm betting that the same thing happens with video games but you only have to send back the CD rather than the jewel case.
That's precisely what they are. Stock returns. But a lot of places are religious about making sure absolutely everything is destroyed so GameStop either doesn't care or someone just threw it out before messing up the cases and such.
Why would you do that? Why would any place anywhere spend even a moment of their employees' time on destroying empty cases? What are they afraid of, this guy? Are there people actually buying cases and discarding the games?
This brings back memories of working at EB Games and pulling things from the trash. My most memorable discoveries included a loose 32X system and a giant Metroid Prime display. For all I know, the latter is still sitting in a closet at my parents' place.