You're just a single planet working on a warp drive. Suddenly a weird looking bald guy shows up and says, "We're from the Federation. We have hundreds of planets and our spaceship full of weapons that can destroy all life on your planet is in orbit. Do you want to join us?"

I'm not saying the Federation would ever do anything, that would be horrible, but the planet is going to say yes because of the implication.

This planet will find out there's trillions of Federation citizens. They'll find out Federation space completely surrounds them, they're all on their own. They'll say, "There's nowhere for us to go, they have trillions of people." So of course they'll say yes because of the implication.

Does the Federation want the planet to join? Absolutely. Is the planet free to say no? Of course and the Federation will respect that and leave. But they won't say no, they're going to say yes because of the implication.