This evil fad started about 30 seconds after the first policeman was hired...
Police have always had problems finding witnesses to crime, this is just a marketing gimmick for rappers and t-shirt vendors.
I meant a mainstream, concerted effort on behalf of many people to not "snitch".
Keeping things from cops started, like you said, about 30 seconds after.
Mainstreaming it, however, is a bit more recent.