On Direct Action: Action can mean a lot of things. Maybe...MAYBE if he’d worked for years to abolish the grading system and gone out and debated proponents of the system and still failed...then maybe his actions might be justified. But he hasn’t done that.
People who act unilaterally aren’t trying to start debate they’re afraid of it.
On hurting the Students: The question was never "is he diminishing the prospects of his students" it’s "is he diminishing the prospects of other students." His students are getting A+ grades so it’s unlikely they’ll be hurt? But what about the person from another university who is trying to get a job where grades matter? What if that person got a B at their university and is competing against one of this professor’s students who got an unjustified A+?
Systems exist to create a scale by which people judge. Even if that scale is as flawed as you contend it’s the scale that exists and he doesn’t have the right to tip it based solely on his own arrogance.