The steel man argument is that WFO means a manager can screen glance as they are walking the floor. That can catch the worst slacking or possible contract violations like outsourcing the work.

If slacking risks randomly getting caught and fired or takes too much work to hide, then folks are more likely to just do the work or quit.

Seeing the screen, or even an activity log, is far less invasive than nearly constant monitoring of the worker's face. That steelman is built for a different debate.