While I agree with this law, there has to be exceptions where there has been a major disaster and people are needed, bad weather is predicted, so extra staff can be put on standby, but if there was a massive car crash on a motorway (highway / freeway) then extra staff may have to be drafted in, granted in those situations people will probably go in to work anyway to help, as they are medics and want to save lives.
However these situations are rare, so would not happen very often, perhaps to add to the above example people should be recognised for going above and beyond in some way,
In terms of working in other work places, if there was a serious issue cropped up for example security breach, or fire in the office, then perhaps the boss would need to contact all employees to update them on the situation. So again in that situation, it would be different.