Bethesda has just offered up their justification for why they are offering up mod creators the option of charging a fee for The Elder Scrolls V: Skyrim mods. Let's see what they have to say.
We believe most mods should be free. But we also believe our community wants to reward the very best creators, and that they deserve to be rewarded. We believe the best should be paid for their work and treated like the game developers they are. But again, we don’t think it’s right for us to decide who those creators are or what they create.(...)

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.

There are many, many more words on Bethesda's latest blog entry but most of what was actually said was already posted above.