You ask that as if the US and Isreali intelligence organizations aren't more closely aligned with each other than they are to their respective countries.
And I bet it's not just those two.
If the CIA had to choose between MI6 and the American people, I can almost guarantee which if those they would pick.
It's very much like cops who will help cover up corruption rather than expose it. Their loyalty to their departments is greater than their loyalty to the people.
They justify it in their head because they think their job is too important, that the department must be protected from the common people who would stand in their way if they knew the truth.
Your point is proven by the local cop who drove drunk in a government vehicle plowed into multiple civilian vehicles and a separate department covered it up. No one heard about it until a year after the incident occurred and it was leaked during the buffalo mayor political race.