Isn't that the first season of the original series that dates back to your parents and grandparents age? there were a lot of changes in the desensitization of our youth.
This change in ratings is due to many generations in which the older more reserved generations died off and those still around pushed for change.
In summary: Movies rated R now would be Rated X in 1968 those Movies rated PG would be rated M which would mean, 16 and older. so If you were 13 you couldn't go to them without a parent. So based on a 1968 Code, Kirk could only do so much in his fight. Kirk would not have been able to bash the lizard's brains out with a rock and show the brain matter and blood spilling on to the ground or sticking to the rock while his shirt was splattered with blood as much movies like the Saw series depict. It was fighting from a different Era of thinking.