Usually a game can be played on a profile other than the original "downloadee" as long as the Gamertag is logged onto Xbox Live. This works on all Xbox Live Arcade titles, but it wouldn't surprise me in the slightest if Microsoft changed this rule for Games on Demand.

On a side note I was at eb games a few days ago to purchase a game on PC and for some reason all there PC games are hidden in a staff only room in the back... lol I found that weird.

The reason for that may be quite simple. Console gaming has all but taken over in game stores, and most of the time it's simply a waste of shelf space. I work in Gamestation (UK) and we no longer take PC games on trade-in or cash for the same reason.