[k00pa]

Major parts needed rewrite to not make it so easy to develop cheats for the game.

If the hackers would have the source, they would not even need to reverse engineer the engine to build their aimbots/wallhacks.

[orbifold]

That does make zero sense.

[scj]

It was a fear years ago when Quake was open sourced [1]. In Valve's case, the programmers probably had to audit all the source code in order to reduce possible exploits.

[1] http://www.bluesnews.com/cgi-bin/finger.pl?id=1&time=1999122...