Pokemon GO uses real world places of interest as locations for PokeStops and Pokemon Gym. And what's more interesting than a cemetery? Dozens of players are reporting that cemeteries, especially those located in major metropolises, are hotbeds for PokeStops and Pokemon Gyms.
While we associate cemeteries with mourning, they were once considered social spots during the early 19th century. Until city parks became commonplace, urban cemeteries were considered cities' primary green spaces and were popular spots for recreation. Recently, some cemeteries have re-opened their gates, allowing the public to use their spaces for reasons besides burying the dead.
In addition to cemeteries, public memorials and monuments are also usually PokeStops or Pokemon Gyms. Pokemon Go also includes a brief history of the memorial or description, so players can learn about local history while catching Pokemon.
Niantic Labs used crowdsourcing from their first game Ingress to determine the locations for their PokeStops and Pokemon Gyms, so any cemetery used for Pokemon GO is probably open for public use. However, it's a good idea to double check before exploring your local cemetery and be sure to respect mourners and give them space and privacy. Also, there's no reports that visiting a cemetery increases the chance of encountering a Ghost Type Pokemon, unless you visit at night, of course.