Isn’t it possible to obfuscate things? It would be pretty strange if UE3 games need the source in near-plaintext to the included in the script packages, just for the game to run properly, that stuff should be compiled.
Then again, UE3 seems to be pretty decompilable in general, you can extract the UnrealScript source code of the Bioshock games for instance with just a few tools, it’s extremely ugly but all the code is right there. And I’m pretty sure they are cooked games and not dev builds.