The Federation makes it quite easy to quit a term of service before completion (even during war time)

I'm fairly sure it's mentioned that once war broke out Rico was no longer allowed to leave, but he didn't pay much attention because his two years weren't up anyway.

once someone has quit they are never allowed to enroll again. This is to ensure that all volunteers are dedicated, whilst also discouraging people from leaving.

On the contrary, the Federation deliberately makes leaving as easy as possible to get rid of anyone who would otherwise leave later, or worse stay and let his squaddies down in a way that would get them killed. You can't just up and leave (though no effort is made to find you if you desert), but at any time you can ask to see a superior, get your papers voided, and walk out off base.

This is because Federal Service is tough and dangerous (by design). It can involve joining the Military, being a Human Guinea Pig, testing survival equipment or Manual Labour.

This is a funny one. On the one hand, in execution it's mentioned those physically unfit to serve in the military do get any pointlessly dangerous job available. But in principle, many speeches are made specifically saying military service is what makes someone worthy of political rights, because of the responsibility of military service. Someone counting the hairs of venomous caterpillars (an example job given in the book) has no responsibilities that could harm or help the country he's serving.