Starship Troopers is a kinda subtle book, and the author wasn't a huge sympathizer of compulsory anything, or of subservience.

It separates civil rights and citizen rights. Everyone has civil rights - no such thing as an unregistered alien, no such thing as forbidding abortions or whatever one wants to do. But citizen status (ability to vote in elections, hold political posts and even work for the government in less important positions, like that veteran with prosthetics in the office who accepted the friends' papers in the book) is something requiring commitment, so it's gained by making a sacrifice in the form of federal service.

It's also not only gained by military service, there are a lot of dangerous and important jobs eligible for that.

Unlike with typical fascism, everyone capable of understanding the oath is eligible for federal service, so if they are not fit for anything military or space related, there still are such jobs to be found.

So IMHO one shouldn't judge it by Verhoeven's movie alone.

Evading service is definitely not punishable there. Except there'll be no second chance to serve.

Is that different from enlisting slaves by offering their freedom?