Using a tool that the other person is not fully utilising is not wrong on its own. In fact, not using it could be considered immoral.
Ok, but here it does not really apply : the tool is the user cpu. If the cpu is used more, it uses more electricity. The user will pay more because someone used its cpu without its consent.
it will not take too much more, having it idle already wastes electricity. Mining with a cpu that would be sitting idle instead of off is less wasteful overall.
The problem is the consent and what it is used for, not the "get your own damn mining rig"