They had to go and ruin it
The locations of all pokestops and gyms were taken from the predecessor game Ingress. The locations and pictures were created / uploaded by players.
This was never an issue before they released Pokemon Go and while it is more popular it also attracts a much younger player base. It seems that these younger players need educating regarding private property etc.
It is against Niantic (the game developer) guidelines to create in game locations / portals, pokestops, gyms or whatever they are also called in Harry Potter on private property.
Quote
'If you come across any of the following, please reject it as a low quality Portal by rating a 1 star for the “Should this be a Portal?” question:
Candidates on the grounds of primary/secondary schools
Candidates on private residential property (including farms)' etc
Full guidelines here
https://opr.ingress.com/help
Also Ingress and Pokemon Go have had warnings not to trespass on the log in screen for as long as I can remember, they rotate them with' do not play while driving ' and some other sensible advice for idiots with no common sense.
Lastly there has been a removal mechanism available before this trial, before Pokemon you could report the locations with Ingress.
https://www.pokemongolive.com/report-location