Saluting is a special thing with the military.
Did you ever see Melania salute the military?
It’s not even required for a POTUS to salute the Military . Reagan really started it. Which then ended up with stupid photos like Obama saluting with the coffee cup and Bush with the dog.
Not denying that it’s special. But, President Trump saluted the Marines all the time. Understandable why Melania would not. Fact is, if Biden was not yet, he soon would be CIC. He would have a reason to salute. And if there was some aid on the sidelines feeding him instructions, there’s a good chance that person doesn’t know or care about the protocols (like Obama and Bush didn’t - to your point).
“Reagan really started it”
President Truman started it.