When Steam is hosting mods for free, then they are effectively just hosting files. They transfer so much data, I can imagine this would be effectively zero cost for them.
Once they start charging for something, the entire payment infrastructure is now part of the picture. They need to pay merchant fees, assume risk for disputed transactions, arrange for the money to be split up and transfered to stakeholders, etc. This may not account for 30% (who knows) but it does make sense to charge something.
Once they start charging for something, the entire payment infrastructure is now part of the picture. They need to pay merchant fees, assume risk for disputed transactions, arrange for the money to be split up and transfered to stakeholders, etc. This may not account for 30% (who knows) but it does make sense to charge something.