Can we drop the preposterous notion that Valve and Bethesda are somehow ripping modders off by paying them? If you think receiving 25% is being ripped off, you're not really familiar with the games industry. Prior to Steam, retailers routinely took 50% or more of the POS value, and then the publisher took by far the lion's share of the remainder. This has been the case in EVERY creative industry for a long time, be it books, art, music, PC games, DVDs... To take a reasonably topical example, George R.R. Martin has a net worth of around $15 million. He's sold 60 million copies of his books. That's 25 cents per book he's ever sold. And I don't know what the last book you bought was, but I'm willing to bet it cost more than a dollar. He's getting considerably less than 25% of the sale value of his work. 25% is a good deal; we get free hosting, free advertising, and we're actually getting a return from our work. I'd take 25% of something over 100% of nothing pretty much every time, tbh.
Likewise, the idea that modders are somehow better off keeping 100% of the proceeds from a donation model is absolute rubbish. For a very long time indeed, my mod was (by far and away) the most popular mod for V2. First-week downloads were around 5,000 per release when it was at it's most popular. I was contacted approximately seven times by people looking to donate (I told them I wouldn't accept anything) in the first 18 months of production. That's 0.12% of the player base who were interested in chipping in a little something voluntarily; admittedly, I wasn't actively requesting donations, but even if you do it's not a good monetization strategy. You need a player base in the hundreds of thousands. Dwarf Fortress, for example, is a full game using a donation model. It combines a huge playerbase with cult status, and only just earns enough money to keep Toady One in a grubby flat in the cheap part of town - and that's with many players offering repeat donations specifically to support him.
Seriously, get real guys. You're not objecting to Valve 'exploiting' modders here. You're objecting to having to pay for mods at all. That's fine, there's a genuine argument to be had over whether mods are a marketable product; they have no real protections from plagiarism and there's no real guarantee of quality control. They often infringe copyright material in a way that simply isn't allowed in a monetized product. They are often abandoned in an incomplete state, or attempt to do things which aren't actually possible with the hardcode of the game itself. These are all important points to be discussed. But don't try and pretend that you're complaining on behalf of the modders because getting paid instead of giving their stuff away for free is somehow against their interests, because it's not a remotely credible argument.