See my post elsewhere in this thread. Its a bit lengthy but I go into some personal anecdotal numbers on Monero mining. Even if the computers only mined 1 h/s that would still net them about $0.01 every 24 hours or so if they had 24 computers (at least according to Monero Ocean).

So if they scaled up... Maybe 10,000 computers, then they'd start making money.

Remember, when you don't mine with hardware you own, you don't pay for electricity.

Now, if its volunteered or don't for some time of compensation for the user (say the UNICEF page accepting browser mining) then I think this is a good idea.

If you just steal people's CPU cycles without consent with this (or even with unwanted ads) then I'm going to frown on that.

Still... It can add up.

And yes... People have gone to jail for unconsented mining... There was an incident in Japan last spring I recall.