In a society as large as ours, I think it generally is strictly smarter. Personally I think it’s detestable for ethical reasons.

well, if he was going to be running a space heater otherwise then this is basically the least harmful instance of mining

That's a pretty inefficient space heater

It's close to 100% efficient in converting electricity to heat.

anything that converts electricity to heat, by whatever means, is essentially equally efficient in doing so.

How is it stealing? He's paying for the electricity, the same price as everybody else who has an allotment in that community.

How is paying for electricity, at the price the provider offered, stealing?

Because the electricity was almost certainly offered for a specific subsidized purpose, likely being funded by a grant of some sort, and thus it is being misused?

Maybe, but then the heating itself would be the problem, not the mining on top of that.