Alright, I am from Lancaster, PA, and here we have a slightly different code of driving than most places I’ve been (including other places of PA). I know of 2 “No Turn From Shoulder” signs within 2 miles from me (but the only 2 I know of) and here is what they are for.
In Lancaster, the locals drive in the shoulder to pass cars turning left (not necessarily at an intersection). This is common behavior from police, school buses, and all the locals and will not get you a ticket. Shoulders are used to turn right when no right turn lane is available. All of this includes intersections with stop signs and traffic lights. There are a few intersections where they don’t want you passing all the cars going straight or left in the shoulder and this is where these signs come from. I live 2 lights from one of these signs and still turn from the shoulder, everyone does. These signs are confusing, not used much, and completely ignored. However, if I saw a cop I would probably not turn from the shoulder, even though they may ignore it.
Hope this helps, and if you drive in Lancaster and you are visiting, have fun and don’t get mad if we honk at you :D