People who are working 40 hours a week have no capacity to take on some important task with no notice unless they go past 40 hours a week. Which generally doesn't cost any extra money.
Which is the problem. What we need is overtime for all. Time and a half after 8 hours in a day. Time and a half after 40 hours in a week. Time and a half after 5 days in a week. Double time on Sunday. Minimum of 4 hours of pay per workday or if called in. Those factors multiply. That's not at all unusual in union shops.