'Piggyback' is a loaded word. Its something we all do when we use a compiler or desktop machine. Yet those folks don't stick their hands into our revenue stream.

Yes.

I paid full price for the game and all of the DLC.

Part of the reason I did this was due to the ability of the game to be easily modded. I knew with modding the lifetime of the game was much greater.

Why should they get 40% of mod sales for doing nothing? They already got paid for making the game.

Now, if they are still providing good customer support with timely bug fixes around the mod system, then perhaps I can see a small portion going to them. Otherwise, why do you think they are entitled to a share of the mod revenue?

The part where they also got to sell the game engine to customers, the developers, and then take a cut of the developers profits too?

You don't buy the mod, and get the game to run it. You have to buy the game, then buy the mod. If they want more money for the game, they can increase the price.

Also, I wasn't aware that Bethesda had released their game engine for free. Care to give a link as to where they've done so? As far as I'm aware you still need to pay for a copy of it.

Microsoft doesn't get a cut of Jetbrains' IDE "mod" revenue, though, despite the fact that Reshaper and similar products "piggyback" (what a loaded term) onto Visual Studio and other Microsoft products. Microsoft already got their cut up front.

Same thing for Bethesda - it's not their value-add and they don't have a moral right to take a cut. On the other hand Steam is providing a value-add here - a market and financial transaction services.