This isn't rocket science... Everyone gets two days off during the week. In operational jobs, they do shifts with either rotating or fixed days off.
L3/L4 supervisors are eligible for overtime. If they work a sixth or seventh day, they can put in a supplemental time card and get the overtime, or if local procedure allows it, they can take it as comp time. Some states require the overtime to be paid out and don't allow comp time as an alternative.
L5's and above are salaried and typically are not eligible for overtime (unless state law specifies otherwise). If they work all or part of a sixth or seventh day, they don't get paid extra.
Maybe you think it's fair to expect management to work over 40 hours a week (or 173.34 hours per month) without any compensation, but I don't happen to share that opinion.
People regardless of pay grade deserve to balance out their time on and off duty.
Come to think of it, if management *were* paid overtime by the same rules as the union, you all would be screaming about how management abuses overtime to increase their paychecks...