Yes they've played this game, they're not going to hire more than they need expecting some not to show up. They know they can get away with hiring just what they expect to need and if most of them quit because it sucks to be a seasonal even more than a new hire they'll work sups because the penalty from grievances is less than the potential overhiring or loss of business. They'll keep doing it as well because the union really doesn't care because in many locations they're still getting money from enough of the seasonals through initiation fees and dues and they get to play the good guy card of fighting for the sup working grievances during peak.
Is it right? No, but nothing is going to change because both the union and the company benefit and it's just the hourlies that have to put up with the results.
They usually have more than they need in the hubs then every day they ask people when they walk in the door if they want to go home and they do. They language in the NMA provides for a guarantee even if you voluntarily go home but people don't exercise their rights. When they go home supervisors work and you blame the union. In every operation there is a percentage of expected call offs, workers comp, disability, NCNS or whatever else. Again it is the obligation of the employer to staff their operation not the union.
You say the union doesn't care because they are getting the dues and initiation fees which is inaccurate in my opinion. If you file a grievance you will get paid if you have a valid grievance.
You are correct in saying nothing will change but it is not because of the union officials it is because certain members allow it to happen and just want to complain. I have won several grievances regarding supervisors working.