Several people have mentioned to me that the book has other ways besides military service to earn citizenship. I did not remember that being the case, so I read it again. What follows is an exploration of what the text actually says and a brief look at the unreliability of memory without once saying “Mandella Effect“.
UPDATE: Some time back it came to my attention that the source of the confusion is Expanded Universe, a collection of essays and short stories from Heinlein himself and from conversations with the author. Heinlein contradicts himself on the matter. For a detailed analysis I recommend the following essay, which covers it in depth.