That’s simple. They always saved Dr. Smith, no matter what he did.
Here’s some trivia for you. The series ran for 3 years between 1967 – 1969, pulled down higher viewer ratings than Star Trek ever did, and out of 84 episodes, it’s quite possible that Jonathan Harris uttered the word “creature” in every single one of them.
When did they leave an “overpopulated” Earth on their voyage? October 16th, 1997. Gulp! 14 years ago.
Kudos also for the premise of colonizing the universe as a solution for overpopulation. An enlightened, Christian humanist approach, compared to abortion, the pill, forced sterilizations, and death panels; especially in light of the myth.