I've never cared for this custom, even when President Reagan started it.
It looks awkward and improper for someone in civilian clothes, even the commander-in-chief, to salute a Marine in dress blues, and it's rarely a proper salute anyway.
It's even more out-of-place when contempt for the military is a core value of the modern democrat party.
The President can recognize his Marine detail without a half-assed civilian salute.