The guys with cameras only show how ignorant and arrogant they are.
Properties fall into 3 categories ( correct me if I am wrong here )
Public Public - like HDB neighbourhood areas , parks , Botanical Gardens etc . Any members of public is able to access the area, unless deemed a nuisance to public and removed by the police.
Private Public - like shopping malls, office towers , private galleries etc , owned by private sector , eg,private limited firms , listed companies or private individuals , but public allowed access to the property ( usually for commercial purposes like shopping, conducting of business with the owners or tenants of the owners ) . And when deemed a nuisance to other members of public, or flouting the rules and regulations set by the owners of the property, they may be removed by the agents of the owners ( aka the management - the security staff in the malls. ) .
Private Private - eg HDB flats, condo units or bungalows etc . No need to explain, if intruders are found in your pte property, you will dial 999 immediately, or in the States , shot the intruder/s 1st then talk later...
I doubt the guys with cameras are locals, they may be foreigners working locally, but paying foreigners income tax rates, thats why they are kpkb-ing. If they are locals, they are a real disgrace . Ashamed to have such people claiming to be Singaporeans.