From what Chesko has described it appears possible that someone could ultimately take another total war modder's work (say, make a half-assed mini mod of DEI or Radious which is little more than just 'throw some Roman gear on Spartan units') and sell it without the explicit permission of the original authors (In this example, DEI or Radious). That is going to dramatically scale back the willingness of people to release open source projects, or damn well any sort of public project to speak of. Even if you are supportive of free modding few people are willing to be ripped off and made to work for another's dime. I already saw people taking my friend Kuauik's work from 1257 without permission. I know beyond a shadow of a doubt they are going to if they could make a buck off it. My own work is absolute dogcrap compared to Kuauiks but I have no interest in wasting hours upon hours of my free time to see some worthless piece of trash come along and get Valve money for my work.
And the absolutely perverse fact is that even if the thief is caught, Valve will not reimburse the customers (let alone the original author). They will quite literally make a profit off a theft they have facilitated.
I will hope the more networked and organized modders than amateur hours like me will be getting in touch with lawyers and determining their rights and valve's liability about. Because it seems absolutely bananas that they believe they can steal other people's work and sell it as their own.
So no I'm not buying a mod and I am not buying any games that promote this.