[Mordekay]

You don't get it.
I don't say that any mod using the source/gldsource engine is non-steam! That is bullshit. Starting with Age of Chivalry over any free mod that used ONLY it's own files and models and materials that come with the FREE games of the orange box is absolutely LEGAL.
Why CSPromod is illegal is the simple fact that it uses material from a LICESENSED game, which is, if you READ the part of the user agreement of the HLDS software and that part that Hawk posted, ILLEGAL. You are only allowed to use ressources from a game you OWN. If you create a mod you have to make sure that ANY part or content of a game you use to create this mod has to be OWNED by the user that want to run your mod.
Do you need CS:S to run CSPromod? Not for the client, but for the SERVER. No server = no game.
And for "that wiki is all he need".
Do you may know tat any wiki page is created by simple users/persons. Any info on a wiki page is only as good as the one who wrote it.
I doubt that anyone would write that usage of his mod is illegal. That would be dumb, wound't it?