No, it's not a marketplace glitch, it's intentional. It's 5.63 GB though, so you need to make sure you have the space.
The Kotaku article speculates that it's free in order to promote the HD remake of the first game, but after Microsoft's E3 press conference I'm wondering if this is instead the first title in this new "two free games per month for XBL Gold members" plan they announced today. Granted, that would leave the question as to why they didn't mention this game in that announcement, and instead focused on Assassin's Creed II and Halo 3, but it was the first thought that crossed my mind.
I didn't know they were doing a promo, I just assumed it was such a bad game they couldn't give it away.