Always puzzled me too this. Year before last I went to the pool local to where we were staying in sud de France wearing my Speedo swimming shorts. Great big Speedo logo right across the arse. Dived in and surfaced to find apoplectic life guard shouting “pas de bermudas” at me.
I’m mean cut off jeans, fine, but outlawing swimming specific shorts is pretty baffling. I’m going to an event hosted by the French version of the CTC next week, and am half expecting a similar reaction by the commissar de roadies to my MTB kit “pas de baggies, et le peak sur le casque”