Having served both as a naval officer and a foreign service officer, I don't share your view that senior field grade and flag officers have "amazing" insight to international affairs. They have their own unique perspective, but so do senior State Department and CIA officers who have spent entire careers specializing in international relations and understanding different cultures.
I am concerned about the fairly recent trend over the past decade of CINCs becoming more influential in our foreign policy. The State Department assigns Political Advisors (POLADS) to CINCs to provide foreign policy expertise and to coordinate actions, but I don't think it works all that well. The military and State/CIA corporate cultures are very different. The military culture is more action-oriented and tends to see things more in black and white than shades of gray. Their decisiveness and confidence appeals to the politically appointed policy makers who want options without too many qualifiers. The can-do attitude works well for military objectives, but it isn't as effective in foreign affairs.
I am sure the opinions and motives of retired admirals and generals fall across a wide spectrum. Many are looking at a second career and what is best for them personally whether it working in the defense industry, serving on coporate boards, becoming a talking head on television, advising a political candidate, seeking office, or being appointed as a government official. For example, ADM Crowe supported Clinton and received an ambassadorship to London. GWB supporter Colin Powell is SECSTATE and he has brought other retired military flag officers to the Department. Prior to that, Powell made millions in speaking fees. General Clark is running for President after serving as a talking head for CNN. Tommy Franks is also now on the speaking tour.
By speaking publicly, General Zinni has inserted himself into the political arena. Nothing wrong with that, but he shouldn't be surprised that his credentials and motives will be challenged.