Although I've never been in the military, I have some basic knowledge about how soldiers are supposed to behave in battle. It doesn't involve staying idle when paratroopers are falling from the sky. A civilian might be shocked and stay frozen in place, but these marines are supposed to be trained, if not brainwashed. The cinematic is visually awesome and the Terran portrayed in the cinematic don't make very realistic soldiers. What, can't the cinematic be great in some aspects and somewhat lacking in others?
Also, as someone said, this is the opening cinematic, aka the beginning of the story. It is not a commercial.
The marines in the Brood War introduction were not from the Earth, the UED refer to them as the colonists. But yes, these were more convincing.