I may be wrong here so please correct me if I am; didn't we use Article 5 as justification for standing with the US when they went into Afghanistan post 9/11? Again I stand by to be corrected but. I think it was the only time article 5 has been invoked in NATOs history. If so, then intervention only if we are directly threatened, does not appear to be immutable - not on our past record. Without question, as others have said the ghosts of our involvement in Iraq are having a heavy influence here.
Article 5, if I remember correctly was invoked by the USA on 9/11 as the RAF deployed an E3 to the States.
Collective defence
The principle of collective defence is at the very heart of NATO’s founding treaty. It remains a unique and enduring principle that binds its members together, committing them to protect each other and setting a spirit of solidarity within the Alliance.
This principle is enshrined in Article 5 of the North Atlantic Treaty. It provides that if a NATO Ally is the victim of an armed attack, each and every other member of the Alliance will consider this act of violence as an armed attack against all members and will take the actions it deems necessary to assist the Ally attacked.
NATO invoked Article 5 of the Washington Treaty for the first time in its history following the 9/11 terrorist attacks against the United States.
From the NATO website.