In France, mealtimes are about community and sharing as much as they are about food, so appreciating every bite is standard practice, even when it comes to the salad.
When eating a salad, the fork is meant to do all the work. If you encounter a large piece of lettuce, you are expected to fold it into a smaller piece and place it in your mouth.
Cutting your salad gives the impression that the host didn't prepare it correctly, and you must "fix" it. It is acceptable to use bread to push the leaf onto your fork.
This rule may also be a nod to when France's aristocracy would avoid cutting the lettuce so the vinaigrette wouldn’t tarnish the silver knives.