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.
'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
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.