I guess it's like having to wear a tie, not much sense in that either.
When you think that in France and Spain it's quite usual for women to have to walk past the urinal to access the toilet, it doesn't bear much consideration on the 'decency' level.
Maybe they just want to keep up appearances ('scuse the pun), as in having to wear a tie, and it's nothing to do with any other motive.
There again, it does avoid any discussion on what is beach wear and what is acceptable shorts.