I am assuming it is just a random police officer reading Reddit on a personal basis and deciding to act on evidence of a crime (either because they are the local force or by passing it on). Police aren’t ever really off duty in this sense.

Why assume a surveillance mechanism when police are individuals who also use the more popular sites?

It's not assuming a surveillance mechanism, it's _describing_ one. Individual police keeping their eyes open _is_ a surveillance mechanism. Admittedly, just about the most banal one possible, but the anti-social behavior it's catching is also the most harmless. But the bad law does actually make it a problem.

Public racist abuse of a dead victim is the most harmless?

Well it's a take, I guess. Go with it.

Oh you are among the 'words are violence' folks?