From Pennsylvania vehicle code:
(d)
Driving in right lane.--
(1)Except as provided in paragraph (2) and unless otherwise posted, upon all limited access highways having two or more lanes for traffic moving in the same direction, all vehicles shall be driven in the righthand lanes when available for traffic except when any of the following conditions exist:
(i)When overtaking and passing another vehicle proceeding in the same direction.
(ii)When traveling at a speed greater than the traffic flow.
(iii)When moving left to allow traffic to merge.
(iv)When preparing for a left turn at an intersection, exit or into a private road or driveway when such left turn is legally permitted.
(2)Unless otherwise posted, no vehicle or combination over 10,000 pounds may be driven in the left-hand lane of a limited access highway having three or more lanes for traffic moving in the same direction except when preparing for a left turn at an intersection, an exit or into a private road or driveway when such left turn is legally permitted
You are also supposed to get in the left lane if emergency personnel are on the shoulder which is defined loosely enough to mean virtually any official looking vehicle on the shoulder.
I'd be shocked if this wasn't the law in all 50 states. As you say it's not enforced, but it is slightly more explicit than 'obstructing traffic'.