How is it that games like watchdogs 2/destiny can allow ppl to be in your game when you don't have ps+ ?

Currently playing watchdogs 2. Sometimes other players will drop into my game.i remember also seeing other players in destiny and I dont even have or use ps+. How does this work? Even on my kids ps3 he has ppl drop into his game on motorstorm apocalyptic. I noticed when I was playing with him the other day