It all comes down to Permissible Purpose
A P.I. license is what makes it okay for a private investigator to sit in his car and video record a person he’s investigating. It also makes it okay for him to surreptitiously follow someone (shadow) around as they go about their daily activities. These activities would be considered stalking were the private investigator not licensed and carrying out his work with a permissible purpose.
Once an investigator is clear that a permissible purpose exists to investigate a person (or put a GPS tracker on a vehicle), most responsible private investigators will take the step of ensuring that there aren’t any court-imposed restraining or protection orders in place.
In recent years, most states have applied and extended their current stalking laws to the use of GPS tracking and this approach actually makes sense.