Thanks for the response, Bepbo.
My 360-S is an American unit, and it in Japan.
Dead Rising Case Zero is digitally distributed with a physical location check gateway, based on the console's IP address. I'm assuming this is due to the censorship laws in Japan against beheading characters in videogames, even in CERO Z-rated games.
...come to think of it, this may be the main reason Case Zero is Xbox exclusive; the PS3... wait, no, the PS3 also does IP-based region locks. I recently tried to buy a movie from the PS3 store just to see if it would. Hm.

The lock presents itself when clicking "Confirm Purchase" at the marketplace dialog. Something along the lines of
"The selected content is not available in your region" appears. This IP-lock seems completely separate from the traditional Region Lock which checks the unit's numeric region against the game's same value. This IP-lock was originally used for non-game media (TV, movies, music) but it has been applied to game content.
They're also using this IP-detection in order to show or not-show some content, e.g.
1-vs-100 had a visible schedule, but the actual screen to download the game was hidden from access, and some XBL contests were shown in the channel but would not progress to sub-screens when selected.
As for the XBLA game, I think it's going to come down whether or not I think it's worth it to spend $5 or $10 just to find out one more piece of information about the IP-blocking system.