The difference is that your average crackhead/tweaker copper thief knows a sketchy scrap yard that will pay them a discounted rate for the copper cable and not ask any questions. It’s unlikely they know where to unload stole fiber optic cable.

The sign is less to discourage theft than to prevent damage by copper thieves who are unaware of the difference and attempt to steal the cable.

Copper cables are easier to reuse or sell as scrap due to the intrinsic value of the metal value and simple structure. Fiber optic cables are harder to reuse because they require precise handling, expensive connectors, and special training and equipment to splice together properly. Unless thieves steal pre-terminated fiber and handle it with extreme care or take entire spools with a buyer ready, fiber is essentially worthless to them since it can't be melted down and reused like copper.

That assumes you can sell it. Chances are it will be worthless once you remove it.

Maybe they're harder to resell illegally