Most parking areas, while public, still post speed limits (5-15 mph) for patrons to abide by. Someone backing out of a parking space is expected to give the right of way to traffic in motion.

In oregon, and i'm sure it many other states, normal parking lots are not considered private property. They are considered "premises open to the public". While the property may be privately owned, if a reasonable person has a reasonable belief that they can enter and use a parking lot for business, its no longer a private asset and the law extends some specific protections into these areas which pertain to general safety for the motorists using them.

If you paved a field and had it fenced, or otherwise signed that it was private property and therefore a reasonable person wouldn't have an expectation that they could enter and use the property then it would be actual private property, but otherwise parking lots are a premises open to the public.

In oregon, standard traffic law does not extend to premises open to the public, things such as stop signs, posted speed limits, traffic flow markers, etc., are all completely un-enforcable by the police. The jurisdiction for criminal traffic law (reckless driving, duii, criminal driving while suspended, etc), does extend to premises open to the public and can be enforced.

So basically, unless the driving observed could be considered careless (endangers or would be likely to endanger any person or property) or reckless driving (endangers the safety of persons or property), it can't be ticketed by the police.