O Yes, such kind of boards are often seen in parks and gardens.
O Yes, some people pluck flowers even when such a board is present.
O They do this because they do not respect the rules and think that plucking a flower does not cause any harm to the place. They may even do this for fun.
O No, people should not pluck flowers.
O If everybody starts plucking flowers, then there would be no flowers in the gardens and the parks. Without flowers, there would be no beauty and no colours in such places. Also, new flower plants would not grow.