I will never understand why developers/publishers put protection on demos. Demos are free and meant to be distributed so what's the point?
As I understand it, it's because Starforce becomes part of the code of the game, as opposed to just a CD check or whatever. Chances are in the demo that it is the same part of the code as in the full game. If you don't include Starfore, it is immediately obvious to hackers where the code is by just looking at the difference between the two.
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This post is shamefully brought to you using someone else's unsecured wireless network