You're reading it correctly and it's working as intended. The people who should have reasonably expected that their job involves overtime are the ones who shouldn't be able to demand punitive extra pay when it turns out that their job involves overtime.

Maybe it sounds more palatable said the other way round: the people whose job can't reasonably be expected to involve overtime are the ones who most need statutory protection when they're suddenly asked to work overtime (because they didn't plan for it when negotiating their contract, for example).