I've seen this a lot, and I was surprised to see a lot of Marines doing it at the Miramar airshow. Thought you guys were supposed to set the example on stuff like that.
Last note, drop salute. Sounding of Carry on..... carry on.
It may be that someone just instructed them to do it this way. I used to do it this way (cut at carry on) because my chief at OCS taught it that way. Then when I noticed others doing it differently, I looked up the OPNAVINST for it.
"When covered, they shall come to the salute at the first note of the anthem, and shall remain at the salute until the last note of the anthem."
OPNAVINST 1710.7A - 1205