So there is (or rather was) a protocol in which civilians did not salute military personnel, up to and including the President of the United States. It was a symbolic protocol, emblematic of the relationship between a civilian government's primacy over the military. A way of showing that civilians like the president were in charge, giving the orders to a subordinated military establishment.

Ronald Reagan was the first sitting President to salute a serviceman, and encouraged his successor George HW Bush to continue the practice, which he did.

Some consider it a nice gesture, a way for the President to show support for the troops. Others view it as an erosion of civilian control over the military, or a slight as only uniformed, active duty personnel are entitled to salute and/or be saluted. Others still are indifferent to the practice if they notice it at all.

But Reagan was the first to do it in 1981, and it's been common ever since.