We all seem to be arguing around the same point: If we can manage to get a few people away ahead of the end of the shift and still fulfill the contingency/legal requirements, then the financial cost is nil and accrues goodwill for the times you need it. But it is not a RIGHT.
Avman's last paragraph concerns me. If it means that the inability to offer early goes is affecting cooperation within the core shifts, then it's not management who needs to re-examine their attitude!
Those people who constantly refer to their 'rights' and what the 'legal requirements' as a reason not to be flexible had better think twice about where this could head. You stick to the letter of the agreements and management do the same. Result: Everyone stays to the end irrespective of traffic or staff volumes. Give a little during the day and you might get something back in the evening.
Someone who plans an evening's activities when they are rostered to 2200 needs their head examining!