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.