We don’t know they are useless, that is just the suspicion. The nature of the work is that sometimes output can be impacted by forces outside of their control. If we wait long enough, the pattern will be obvious, but why pay someone to not do work when we can just install software on their computer that will almost immediately let us know that they aren’t even putting in full days?
I honestly don’t get the opposition to this kind of thing. You’re on your work computer, not your own device. Use the work computer for work and use your personal devices for personal stuff. If your contract says you work 40 hours per week, work 40 hours per week.