Legalities aside, You can use compute how you want, that doesn't stop anybody from accurately measuring its usefulness. Given that this is an objective measurement.
I can have a computer on an endless loop without any idling, consuming as much CPU time as it can, I do not know any other classification of this action than wastefulness. crypto mining is useful of course, the end goal of mining is to get crypto, of course, there is no further goal beyond that.
How is it an objective measurement? What is useful and what is not useful? Is Minecraft useful? What does useful mean?
Via comparison of the solutions, we measure objectives and actions performed and the time it requires to do them.
Objectively, Minecraft is a game, an objective of many games is to waste time, therefore this cannot be used as a measurement, but enjoyment is. as a game it fulfills its role. wastefulness could be classified in comparison to other solutions/clones, like minetest. If minetest fulfills exactly the same goals with less action as minecraft, then minecraft is in comparison wasteful.
Another measurement is mining solutions can we achieve the action of mining with less compute? can we not mine at all? does not mining achieve the same goal? if it does, then mining is objectively wasteful.
The objective of crypto is replacing fiat currency, it does not do this, therefore it is wasteful. Of course, this is only true until it does replace it, which is why we must compare, would this cryptocoin scale to the entire world? does it require more work for it to do so? if so then it is wasteful.