I find it interesting that online and brick n' mortar stores are already selling Xbox One games before even selling the system! My local Walmart has the full day one release selection available now. Checked Amazon and about half their Xbox One titles are available.

Anyone have a clue why they decided to do this?

Common practice as far as I'm aware. I guess they do it to hype people up for the new console as well as secure as many sales as possible before the massive crowds and rush that comes with the new console's arrival.

Yep, shared mutual benefit for both the 'launch' games and the console itself.