Miners used to use canaries to test for the presence of deadly gas. Apparently, the ASPCA or PETA have gotten to the US Marines, because instead of using canaries or other expendable animals, they've decided to use their own men to determine whether there are deadly chemical or biological contaminants in the area.
This was just a drill. There was no gas, and everybody knew it. In combat, however, the Marines realize the chosen ones might panic and put up a fight. So just in case, the other Marines always disarm those whose masks are to be removed - by force if necessary.
Jesus muscular Christ.