FAA considers anything above "blades of grass" to be subject to under their authority. There's some question about what makes a given airspace "publicly navigable", but it's generally understood (at least by hobby drone operators like myself) that if there's nothing overhead and there are no tall structures close by, the FAA considers that airspace publicly navigable.
On the 400' rule: Outside of specially-granted permission and certain other specific situations (e.g., around tall buildings), the FAA requires all unmanned aerial systems ("drones", although that includes things like RC planes) to operate lower than 400' AGL within Class G airspace. The LAANC system is a somewhat automated way to obtain permissions in some controlled airspace.
More info on UAS airspace use: https://www.faa.gov/uas/getting_started/where_can_i_fly/airs...
Since the surveillance in this case appears to be from publicly navigable airspace (according to the FAA's definition), I would expect established case law discussed in the aerial surveillance doctrine Wikipedia page to allow it, regardless of the use of a drone. The use of a private contractor might be of some interest in the case, but I'd wager it's viewed by the court as irrelevant so long as the pilot was complying with the Part 107 rules.