I would think it’s the obligation of the massage parlour
I was thinking the same thing. It's like when the police bust and charge people for drinking at an unlicensed bar.. How the heck would I know if they have a license or not? Yes, I know technically all businesses in HK have to display their BR somewhere visible but 2 things about this rational. #1 is can't someone simply print a fake BR quite easily? And #2 who actually walks into a massage parlour or bar and goes like "Can I see your license first?"
But this guy was probably just mongering. And to that... Who cares? Why do people here (or I guess anywhere but I feel it's more accentuated in HK) love "scandals"?