I can second this. Also most, not all the time but most of the time the park extention occurs prior to the operational day and there is a window of opportunity for the cast to know and reach out to their managers if it causes a hardship.Having worked many evenings when my shift was unexpectedly extended longer than originally scheduled, I can assure you that most CM's do not care as it means more money. You may have a few who are, rightfully, annoyed but most know that's how this business rolls.
Likewise on the occasions when park hours have been extended day of or in the middle of a shift, there's usually some wiggle room and the ones who truly cannot stay can be released. The others are typically more than happy to take the overtime.
It's really not that big of a deal. The mood of the cast suddenly does not turn grumpy because the park hours have extended.