The modders were getting 25% of the mod sales, Valve was getting 30% (a standard for digital distribution channels), and 45% going to Bethesda. The outrage being how much Bethesda got. Bethesda responded to this in the linked article [0]:
> Many have questioned the split of the revenue, and we agree this is where it gets debatable. We’re not suggesting it’s perfect, but we can tell you how it was arrived at.
> First Valve gets 30%. This is standard across all digital distributions services and we think Valve deserves this. No debate for us there.
> The remaining is split 25% to the modder and 45% to us. We ultimately decide this percentage, not Valve.
> Is this the right split? There are valid arguments for it being more, less, or the same. It is the current industry standard, having been successful in both paid and free games. After much consultation and research with Valve, we decided it’s the best place to start.
> This is not some money grabbing scheme by us. Even this weekend, when Skyrim was free for all, mod sales represented less than 1% of our Steam revenue.
> The percentage conversation is about assigning value in a business relationship. How do we value an open IP license? The active player base and built in audience? The extra years making the game open and developing tools? The original game that gets modded? Even now, at 25% and early sales data, we’re looking at some modders making more money than the studio members whose content is being edited.
> We also look outside at how open IP licenses work, with things like Amazon’s Kindle Worlds, where you can publish fan fiction and get about 15-25%, but that’s only an IP license, no content or tools.
> The 25% cut has been operating on Steam successfully for years, and it’s currently our best data point. More games are coming to Paid Mods on Steam soon, and many will be at 25%, and many won’t. We’ll figure out over time what feels right for us and our community. If it needs to change, we’ll change it.
If you ask me 45% is too high, I don't think it should be exactly 0% but to give the developers so little seems criminal...
Another source for this was fact that there is no real support for mods especially with mods interacting with other mods. While there was a return period (24hrs IIRC) a number of gamers were arguing that you don't always find out within 24hrs if a mod will conflict with another mod. Maybe a long trial period would have helped to alleviate some of this.