Why is ozone illegal?

In the realm of environmental science and policy, ozone often holds a complex status. Could you elaborate on why ozone, which plays a crucial role in our atmosphere's natural protective layer, is sometimes deemed illegal? Is it due to the harmful effects of ground-level ozone, which is a pollutant resulting from reactions between nitrogen oxides and volatile organic compounds? Or is there a different context in which ozone is regulated and considered illegal? Understanding the nuances of ozone's regulatory status would be invaluable in informing environmental policy and practices.