I'm trying to talk myself out of this perspective I have here. If you can help me with that, please do.

How about how it asks me to pay every time I download a mod, and makes me download each one in a collection even though they have a one-button feature? Same results, same total download size, but they turn it into a demeaning menial task. The only reason I can imagine for it is that they want non-paying customers to get bored and quit, cutting down on total downloads.
I can't really tell what value they're adding, since it's the modders and the collection indexers who do all the actual work to make the mods and make sure they work with each other. So they're a hosting service. And they provide... discoverability? But I've never discovered anything through NexusMods, I only used it after finding out that NexusMods was the only place I could easily get the mod collection I already knew I wanted. But it turned out it wasn't easy, and that that was a deliberate design decision to try to squeeze me for cash to make their manufactured inconveniences disappear. And that is something that I oppose on a moral level.
"But they provide the app that places the resources and they swap the executable out so it still launches through Steam!" No, SMAPI does that. It's the thing that places and runs SMAPI, I guess.