Seen it on a dock on Watts Bar in Tenn., but nowhere else. Funny thing is ... if he's there he'll tell you about a brush pile just out from the dock, in deeper water (& I always found fish on it).
Basically it's to discourage "dock shooters" from potentially damaging the boats, boat covers, or other items that an angler's bait may errantly come in contact with. That would include Bass anglers, as well as Crappie anglers.

It's not legal to get out onto a private dock ... that constitutes trespassing. But I doubt they have any legal rights to the waters around the dock, just not sure about the water "under" it. Your local F&W office can answer that question.