Garnet: "Alright Steven, you are forbidden from sleeping until we find Malachite. To help you with this, I've contact my associate, Mr. Krueger."

Personally I think lapis knows that she has basically nothing to go back to, she doesn’t care about earth she hates the crystal gems(besides Steven obviously) and would probably be executed if she tried to return to her home world, so I think she’s fine with being imprisoned forever since at least then her life has a purpose.