(IANAL)
The key phrase to privacy laws has been and continues to be "reasonable expectation of privacy". Courts have held many times that if is is visible to an average height human going about their daily activities, it cannot be considered a "reasonable expectation of privacy".
So, in this case nothing serves as a screen to that visibility. Had there been a 6' high SOLID fence blocking view, it could be argued that taking to the skies for observation was a violation of a reasonable expectation of privacy the fence creates. However, there are cases where the court has held that neighboring homes' second floors are reasonable observation points, so it's still a grey area especially if the drone remains outside the property lines of the observed parcel.
Just my .02, but the standard is actually quite high and pretty established in most locales in these United States.