Why do you think that? Compared to ads that are heavy on the user's network and CPU, track them, get them tons of unwated spams and other shaddy stuffs; why do you think giving some CPU power for a background miner is any worst?

When I did the experiment I thought it was a good deal for users.

If you can't see the difference between showing an ad and mining bitcoin using your visitors' computers without their knowledge, you deserved to lose half your visitors.

Who said it was without their knowledge? It was clearly stated in my TOS and I even sent a newsletter about it.

Also I had a (sort of) premium plan that allowed to pay a small fee (2€/month to remove the ads and get some extra featues. When I made the BTC mining experiment they still had the option to choose between the free plan (BTC mining) or paying 2€ to get it removed.

Hiding something in things you know people don't read doesn't mean it's suddenly an 'OK' thing to do.