[quote name='JKSonic']Care to share that screenshot where it literally says "or the Microsoft Store"?[/QUOTE]
It advertises the deal at the bottom of the Microsoft Store on this link.
"Purchase all 5 Summer of Arcade games: Bastion, From Dust, Insanely Twisted Shadow Planet, Fruit Ninja Kinect, and Toy Soldiers: Cold War before August 23, 2011, 11:59 pm EST, and receive Crimson Alliance and three characters FREE! Limit one (1) gift per person. To qualify for your gift, you must purchase all 5 games from Xbox LIVE Arcade using your Xbox LIVE account by August 23, 2011, 11:59 EST. The download codes for your free copy of Crimson Alliance and free characters will be sent to the email address associated with your LIVE account after the end of the promotion period. Please allow up to 2 weeks for delivery on or around September 7, 2011. "
I'm assuming that no matter how you obtain all five SoA games, if you do indeed have all five on your profile they are just going to email you a code for Crimson Alliance.
I guess we'll see but I don't imagine anyone who bought Bastion through this (MS Owned) website will have any issues with the SoA promotion.