Teams be in damned if they do, damned if they don't situations.
Cuz if they tell a player any time their name comes up in a possible trade, lord knows the player may pout, sulk, threaten to sit out, half ass it on the court, run to the media himself, etc.
I think the reason why after every trade there's ALWAYS a "Player A found out he was getting traded by Twitter" is because teams rightfully wait till all the t's are crossed and i's are dotted before informing their players of the trade. They can't help it that leaks and rumors run rampant and that some of these reporters have sources within each team that spills the beans to them or the agents.