It's illegal to bring some across the border but there are no border checks. And it's different from the illegal market as well. People can just buy from legitimate businesses, cross the border and re sell it in France. The market is gonna be flooded with high quality cannabis which will profit a neighboring country, it's not a good position to be in.
France has already been flooded with legal hemp from Switzerland which has created many issues for authorities. The government knows it's a huge market particularly for a large agricultural country so yeah they'll have practically no choice but to legalise (although it seems the French gov has been more open to it lately)