They’re carrying M14s which was the issued rifle when I entered the Corps in 1969. The M16 was issued in the combat zone at the time. I would think that these are parade rifles since they look more like the public would expect in a parade formation or drill event. The Marine Silent Drill Team still uses them I believe.
Now, why would they have the bolts removed? It could be because they don’t trust the troops. You can still obtain 7.62 rounds and there is no telling what stupid things a young marine might do.
With all due respect, Sir, those are clearly M-1 Garands.