To be honest, I actually think this is a fairly minor symptom of a much bigger issue. In fact, taken on its own merits, I actually quite like the idea, and it's the wider implications of this, rather than the principle behind it, that I take issue with.
While we're certainly going to be seeing a lot of mods going premium-only in the near future, I honestly expect that a majority of mods are going to remain free. Now, back in the days of Reach, I was something of a forger. I was never particularly good, but that didn't stop me from building maps anyway, and even though there were tens - perhaps hundreds - of thousands of fantastic maps and gametypes being produced by the community, I still enjoyed Forging simply because I wanted other people to play them. There's something very affirming about the idea that other people are enjoying something you've created, and while it no doubt sounds vain, I loved the idea that sometime, somewhere, someone would start up a game with the words 'hey, this map is by RSR, he's a great forger', or whatever. The fact that you've never heard of anything I've made, or that I was ever even a Forger at all, tells you everything about how well that worked out, but oh well. Anyway, the point is: while it would have been nice if the people who'd downloaded and played my maps had offered me some sort of payment for them, be it a few cents or a few dollars, what was far more important to me was that people would play my maps at all, that I'd get all the recognition and satisfaction that comes with other people appreciating my work. This is basically a very long way of saying, while a lot of modders no doubt will jump at the chance to get paid for their work, I genuinely expect most of them will keep theirs free simply because it increases the chances of other people actually downloading and playing with them, and that's more important than money for a lot of creators.
And even if that's not the case - I still don't really see the issue. If someone wants to invest their time, effort, and creative energies into producing something for the purpose of making money, how is that different from any other creative endeavour like writing a book, recording music - or even making a videogame? We're happy to pay for all of these things, so why not for a mod? As ever, it's a case of supply and demand, and the consumer is of course free to not purchase mods if they object to the pricing - and unlike things like day-one DLC or paywall-locked content, you're not actually losing anything from the game itself if you choose not to, but instead you're just not getting bonus stuff. Think of it like going to the cinema: while it might have made the experience better, you're not actually going to miss any of the film itself if you don't buy popcorn because it costs extra rather than coming free with the ticket.
More importantly - who are we to dictate what someone is and isn't allowed to do with their own content? if someone feels that they've created something worth paying for and tries to sell it, then either the consumers will agree, in which case they buy it, or they don't, in which case they don't. There is absolutely nothing wrong with creating art for the sake of making money - provided that the consumer feels that the product was worth how much they paid for it (be it $60 or nothing at all), should we really care about why it was made?
No, the real issue here is the shocking percentage of what actually goes to the modder. 25% is an appallingly low number, and frankly, I don't understand why it's so little. I'm not sure where the other 75% is going to go - between Bethesda and Valve, maybe? - but this alone is enough to put me off purchasing mods via Steam. I don't have any objection to paying for my mods - in fact, I'm even happy to pay a little extra to Valve for use of their servers and bandwidth - but I'm not willing for so little to actually go to the persons responsible for creating my content. At the very least, the figure should be 35%, which is in my mind acceptable as it allows for Valve's traditional 30% fee, and the remainder then being halved between Bethesda and the modder: this is still lower than I'd like, but it would at least seem reasonable. The real worry here is that Valve is getting greedy - and given that they're essentially the premier PC game distribution platform these days anyway, there's a real concern that in cornering the market on mods as well, they're going to establish a monopoly, which is never a good thing for the consumer.