Why is that crucial? If a unidirectional bezel is knocked and accidentally moved on the dive, it could only display a longer underwater time. Which means the wearer will overestimate rather then underestimate immersion time, As well as in cases that need decompression stops, this can be a make-it-or-break-it factor https://www.watchesworld.com/frozen-in-time-luxury-watches-for-winter/