Because there are allies and there are allies worthy of knowing about the whereabouts of your nuclear submarines.
It's to people in this case. Revealing information to allied countries has to be done in a controlled manner where the consequences of any such reveal are weighed with the interests of all the relevant parties taken into account. Here, it's just Trump shooting off his mouth to the leader of a country that has no interest in the North Korea problem to begin with. Moreover, Duterte isn't wholly aligned to the US to begin with, and he can potentially turn around sell this information to China or Russia.
It's in the article: the leaker is Trump!
The latest events point towards the contrary.
It did come from the Philippines, but Trump officials are basically lining up to confirm that the details are correct.
Telling Duterte random military secrets isn't going to get on his good side. It's only going to make him think that you can't trust Trump with secrets.
Why would it break the alliance? The US already knows that that kind of information shouldn't be shared with Philippines and countries do things that their allies aren't fond of all the time. In general, the US wouldn't even know it happened.
Maybe someone should tell Trump that!
It almost seems that the President should be sharing important information like that using a different channel. And that more care should be taken as to whether to share it at all, or at least more than, "Hey do you know how awesome we are? Get this - we have two submarines..."
I think you should really pick a good book about the Cold War.
Countless nations across the world seemingly aligned with America have been dealing with its enemies under the table out of convenience. Most times the CIA only caught a whiff of it when it was too late. Some of them went as far as to outright side with Russia or other rival nations/actors when push came to shove. It continues to happen, as seen in the Middle East and Asia.
The most idiotic thing about Duterte is that there's a very good, well known precedent in the region of a leader posing as an ally/neutral actor and then utterly fucking over America.
I've recommended this book a number of times over here.
The chapter on Indonesia is one of the most bewildering parts of it because it provides an extremely clear and well researched look into how America utterly screwed the pooch by trusting Indonesia's Sukarno, only for him to turn heel at the first chance. It was a veritable cascade of fuckery that only came to happen because of poor leadership and the complete divorce of the CIA from the White House.
Not all allied nations are equal. Not all confidential information is on a need to know basis with even our closest allies. Only South Korea should be privy to such info since the other countries are close with China afaik.
