It's an internal cultural matter. It was said in the episode that Moclas was a very harsh planet and the Moclans have only managed to survive by following strict cultural rules. They don't want inflict them on other species but mearly want ensure Moclans are still around in the 26th century.
I think it's also important to see this from the Moclan perspective. The Union is very different from the Krill. They are probably more like NATO to the Moclans while other Union planets are like a ethical alliance. For the Molcans they get an economic boon and security.
It's also an interesting rebuttal to the somewhat anti-theist stance of the show (I say somewhat) in the fact the Moclans are every bit as repressive as a theocracy but are secular so it hasn't changed. You don't "outgrow" ideology that doesn't make sense.