Is there a law that requires red tail lights?

In the vast majority of cases in many countries 'YES'!

This is because it diffrentiates which end of the vehicle is which at night or in low visibility. Also the most important is the fact that if you are traveling behind a vehicle without this at night, it makes your job of distance judgement a whole lot like a 'piece of cake'.

The only example I know of where you are not required is in some rural areas if you are driving a agricultural vehicle but even then some of those alternative vehicles are not permitted for night use.