The idea is that the Leftists would never make it anywhere else. They would taste the vacuum of space long before they reached the destination. When air has a price, it isn't as likely to be wasted.
Leftists don’t have the math to get a foot off terra firma, let alone the ballistic cAlc to navigate to Mars.
“They would taste the vacuum of space long before they reached the destination.”
Great, then you get a decade or two of peace before the next generation grows up, and then what are you going to do? Execute half of them if they start espousing the wrong ideas?