It's important to take into account the circumstances of a situation. In times of war, different standards are often applied to domestic and foreign policy as countries are working in their best interests to defend their home front. In terms of the Zimmerman telegram, I think it was a strategic move for Britain to not reveal its contents to President Wilson. Had Britain not kept the information in the telegram a secret, Germany would have discovered that their communication system had been cracked. As a result, they would have come up with a stronger, more secure, method of encryption, making it much harder for the Allies to intercept their messages. In turn, it is possible that the Great War would have continued for much longer, and more lives would have been lost in the end.
Despite the United States claiming a policy of neutrality, it was obvious they were supporting the Allies. The British knew the Americans were on their side. Because of this unspoken alliance, I believe that if the United States were truly in a path of immense crisis presented by unrestricted U-boat warfare, the British would have notified President Wilson. Because the dangers presented weren't catastrophic, it was a good choice for the British to keep their knowledge to themselves.