It’s understandable that in order to relax, users need to know they’re not being overheard – though there is a less playful side to this. If groups are perceived as a place to say what you really think, away from the constraints of public judgement or “political correctness”, then it follows that they are also where people turn to share prejudices or more hateful expressions, that are unacceptable (or even illegal) elsewhere.
The idea of public judgement or "political correctness" here is exactly the definition of a modern digital pale, around which technology is being used to extend people's thoughts.
