Actually, the law does require pedestrians to walk on the left side when they are in the roadway. Here is the relevant Pennsylvania statute, for example:

§ 3544. Pedestrians walking along or on highway.
(a) Mandatory use of available sidewalk.--Where a sidewalk is provided and its use is practicable, it is unlawful for any pedestrian to walk along and upon an adjacent roadway.
(b) Absence of sidewalk.--Where a sidewalk is not available, any pedestrian walking along and upon a highway shall walk only on a shoulder as far as practicable from the edge of the roadway.
(c) Absence of sidewalk and shoulder.--Where neither a sidewalk nor a shoulder is available, any pedestrian walking along and upon a highway shall walk as near as practicable to an outside edge of the roadway and, if on a two-way roadway, shall walk only on the left side of the roadway.
(d) Right-of-way to vehicles.--Except as otherwise provided in this subchapter, any pedestrian upon a roadway shall yield the right-of-way to all vehicles upon the roadway.