That feels like the problem to me--there's no incentive for a website to do so instead of simply mining without the user's consent.

Except being morally objectionable and a PR disaster? No serious website would do that. I've completely stopped visiting The Pirate Bay after they put a miner there without users' consent.

The serious use case for these miners is that you have some kind of incentive for your users to mine, not just have it mine randomly in the background. I guess it works for pure donations but take a look at "Use Cases" on the front page for a couple of examples of what I mean.

That makes sense. I do appreciate that they went to the effort of putting intended use cases on their website, and I wouldn't mind explicitly encountering some of them (I would gladly mine some cryptocurrency to help support video creators I enjoy rather than watching or seeing ads). I just worry that "morally objectionable" and "PR disaster" aren't enough to stop JS miners from being used without the user's consent on sites--to me it feels somewhat comparable to ad tracking behavior.

I feel the same way about 'modern' websites that are written as JS-only 'web apps'.

And the big problem isn't consent. If I'm paying to watch a video by mining, my battery might not last the duration of the video I'm mining to watch!

Okay I'll bite. How is your choice of device the dev's responsibility?

I mean it's not like it'd make sense to flame blizzard or bethesda because you can't play as long as you'd like on a laptop/mobile.

You use the proper device for the proper use, and this way you just get an additional option to trade value with the site owner. You're free not to mine in the same way you're free to get a paid account if you don't like ads.

That's not "the big problem" in my opinion. I don't even know if I would consider it a problem at all. If I want to drain my computer's battery and get something for it that's my choice. As I said in another comment, the actual big problem is rogue sites putting this crap without user initialization or even informing them.