I sadly finally understand the difference between free beer and free speech.
Suppose I want to use Mouse Guard rules (simplified Burning Wheel rules) to run a campaign set in the time that the Kang were overthrowing the Quan Empire. (I absolutely love Talislanta so I think about stuff like this.) Suppose I am so proud of all the prep work I do for the campaign that I want to put it on-line for others to appreciate. I am sadly not allowed.
Actually, I might be able to get away with it for two reasons. 1) People put up gaming websites for individual campaigns all the time, and those sites are really just for you and your friends to have a central place to keep game notes, so a website like this, even if it is a drupal social networking site or wiki, could constitute "personal use". 2) SMS is just a cool dude, from everything I've read that he's written or that's been written about him, so it's unlikely he'd get on my case about it.
But Sanglorian is right, the awesomeness of a website with the entire Talislanta collection on it would be even more awesomepants, were it not tinged with that un-open CC license.