When's the last time a trade you heard leaked through the media actually became true? There are going to be some exceptions, such as trade deadline approaching and team moving a guy who isn't going to re-sign; or player X being adamant about going to team Y (*cough* carm *cough* elo).
When a trade is being discussed, only a small group know. And let's see do you think league GM's or player agents or coaches or owners are tweeting status updates? Since so few people are in the know, it's wouldn't be too difficult to determine who leaked and you'd lose your credibility.