Except where companies explicitely allow them to build MODs.

In the case of Command & Conquer Generals I read that EA fails to produce a new patch for fixing exploits and weaknesses - instead, I read, they were "officially supporting" a group of MOD-Makers who were buildinmg a MOD that does just this.
Fine for EA : More money saved. <img src="/ubbthreads/images/graemlins/badsmile2.gif" alt="" />

And, by the way, NWN is a case of its own. It's designed to do nothing else than develop MODs. <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />

When you find a big kettle of crazy, it's best not to stir it.
--Dilbert cartoon

"Interplay.some zombiefied unlife thing going on there" - skavenhorde at RPGWatch