Those were students, I've seen plenty of licensed pilots do the same which is why I teach hand on the throttle below 1000ft, it's only really for 2 minutes max and there's nothing else for your hand to be doing so it might as well be making sure the throttle is fully forward. It really does creep back in the 152/172/182 not so much in the pipers but it's good technique.
In my cub there's also a tendency to hit the rear throttle with your elbow when you trim and there have been a few anecdotes of "presumed" engine failures due to the throttle being nudged back with the elbow - little pathetic but it's a definite possibility.