It should have been the other way around with that philosophy. Valve getting a 25% listing fee is more than fair for not having to do any work. Not every mod is going to sell thousands of copies.
To play on a conspiracy theory. Valve would take 75% rather than 25%, keeping even the best modders from getting rich off their own work. The 75% was part of an evil scheme concocted by EA and Activision to make sure these modders couldn't start their own upstart independent game developer by racking up funds on successful mods. Out of the 75% of the mod proceeds, Valve really only got 25% of the profit and EA or Activision got the other 50% of it depending on who published the game getting modded.