The copy protection does not always check the same thing. Copy protections can check different things at the disc and they usually try to check if the EXE was manipulated. So it's quite possible that the disc check on launch passes, but other checks fail. F.e. if you're using some kind of live-debugger at you're system, or if your DVD-ROM needs too long to read again (start to spin again) after it went to sleep mode, or if another part of the disc is checked.
I'm not complete sure of course because I'm talking about the experiences with the german version and you're using the english one.
But maybe it would be a good idea to try out the german demo:
Link
Of course you won't understand a thing, but you can check if the intro movie is played and if the camera is in the inventory.
