Wearing a burkini is a crime in France.
France is a sovereign state comprising territory in Western Europe. It is a sovereign state which means that it is represented by one centralized government. The government can make any law and enforce it. France has already banned the covering of the face in 2010, this ban was also applied to the "burqa" which led to a lot of controversy in France.
Here is another case which happened recently.
Related story - 15 mind-blowing facts about France.
A burkini is a swimsuit for women designed in Australia by Aheda Zanetti. The design of the suit is intended to accord with the Islamic traditions of modest dress.