Because 99% of orders to go through launch procedure were drills. and because they could never know if it was a drill until after they completed their procedure, at least for the first missile, getting shot in the head was a deterrent against insubordination.
Actually feeling the rumble of a launch after you turn the key for the 5,000th time was a shit-your-pants experience. It was just a test. The poor guy's CO thought he needed to be messed with a little and did not tell him they were test launching an ICBM aimed at the ocean without a real warhead.
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