It seems like a wink-wink, hush-hush deal struck between Redbox employees and certain Walmart brick and mortar employees. I would venture as far as assuming that the three-copy-per-person limit was am arbitrary decision by another brick and mortar employee -- policies like these usually come from subjective decisions by well-meaning but uninformed managers in the retail world.

There's a similar scenario where the last Blockbuster Video in Bend, Oregon restocks its DVDs by buying at Walmart, but it's detailed in one of the documentary videos about it that the manager of said Blcokbuster has a special off-the-record arrangement with Walmart management.