Uh, because they own the plants which overgrew out onto the beach. Similar principle if you had plants or trees which grow out of your property and end up blocking the sidewalk."These people should be fined every day until they understand that they don't own the beach."
If they don't own it, why should they be required to maintain it?
Also, in the case of sidewalk planter strips, for example -- you don't own them, but you're required to maintain the portion fronting your property.