The Rules of Parisian Parks
Unlike London or New York, where parks are wild sprawls of grass designed for lounging, historical Parisian parks (like Jardin du Luxembourg or Tuileries) are highly engineered architectural spaces.
Pelouse Interdite
In most classic gardens, walking on the grass is strictly forbidden (pelouse interdite). The grass is meant to be looked at, not used. Instead, you are provided with iconic green metal chairs to arrange on the gravel paths.
The Exceptions
Parks in the outer arrondissements designed in the 19th century under Haussmann (like Parc des Buttes-Chaumont or Parc Montsouris) follow the English landscape style, where sitting on the grass is allowed and encouraged.