“Don’t tell anyone because it’s confidential” is inherently hypocritical
A few weeks ago, someone shared something with me "in confidence". He added "in confidence" as a caveat because that thing had been shared with him "in confidence". This morning I shared that thing with someone else "in confidence".
It is inherently hypocritical to share a secret that you received in confidence while asking the recipient not to share it. Why? It's called "passing the buck": If everyone on Earth were to share your tact for sharing the secret, everyone in the world would eventually know it. This is the essence of Kant's categorical imperative:
According to his reasoning, we first have a perfect duty not to act by maxims that result in logical contradictions when we attempt to universalize them. The moral proposition A: "It is permissible to steal" would result in a contradiction upon universalisation. The notion of stealing presupposes the existence of property, but were A universalized, then there could be no property, and so the proposition has logically negated itself.
(As an aside, I wonder what Kant, or someone actually practiced in moral philosophy, would say of my opinion that the only actor who is absolved of blame in this scenario is the person who originally shared the secret.)
So why are we often comfortable sharing a secret that has been passed on to us when we'd be upset if that person then shared it again? It's probably one part combination of Optimism Bias: we're irrationally confident that our moral violation won't be the one that causes a real problem; one part the Bystander Effect: We excessively share responsibility (or blame) with everyone else who's standing by, witnessing the crime; and one part social bonding: we view the person we're sharing the secret with as more important than the person who shared it with us.