Hades, do you ever play basketball with a group of friends? If so, do you adhere to every single rule that the NBA has? 24 second shot clock? 12 minute quarters? foul outs? If not, then would you say you're not playing basketball? I doubt anyone who plays a game of pick up basketball would say that.
My point? Obsidian (and Bioware) borrowed ASPECTS of a rules system to use, but then modified it to THEIR liking. They took an existing rules system to give them an idea of how things would work, then tweeked things to fit what they wanted their game to be. Just like you and your friends took the basic basketball rules, but eliminated or changed rules that didn't really fit with what you wanted to do.