You saying that reminds me that I actually had the exact same problem--but with OS X's built-in unarchiver. Certain demos would not launch after being unzipped.
Then I tried a (free?) util from the App Store called The Unarchiver, just for those few apps, and it solved the problem. (As well as opening archive types the built-in one can't.)
Weird, but the answer seems to be: try a different unarchiver! Luckily, this issue has been rare.
(EDIT: I suspect it's caused by a few developers, by chance, using a zip maker/settings that the built-in Mac unarchiver doesn't like. Maybe certain Windows-based tools--although I get tons of Windows zips and never have the problem. So it can't be any common tool/settings.)