Why is it only vessels over 24m that the rules apply to?
Answers and Comments
24m is the threshold used to differentiate professional and non professional vessels in the French legislation.
There are indeed many more vessels under 24m (the average is around 10m around). However, the impact of small units is less important. They destroy mainly leaves, and do not impact the dead meadows.
For more information on the impact of anchoring: https://www.youtube.com/watch?v=yhRdrBVHk-c&t=1s
However, the repetition of anchoring of small units can also have some long term impact on the dead meadows. Some Posidonia areas are banned from anchoring for all types of units and many eco-mooring projects are under development.