New Zealand employment laws around Public Holidays got a rehash a few years ago.
The changes saw that employees had to get paid time and a half and a day in lieu. There really wasn't a norm before then. I signed a contract that had me earning 3 times my normal pay if I had to work on a public holiday (of course, this NEVER happened. It wasn't likely to ever happen).
When this happened, cafes and restaurants started to charge a 15% surcharge on public holidays.
Imagine it... they have a choice to open their doors or not open their doors. The cost of opening their doors is that they have to pay extra for the employees. The risk of not opening their doors is that they lose potential trade.
If the cost of opening their doors is outweighed by the potential trade, then surely, the cost of those employees has already been taken into account. Otherwise, it'd be a no-brainer. Don't bother opening the doors. Go to the beach instead.
It's the cost of doing business. So then, why should they slap on an extra 15% on top?