Only people who have completed a term of service are eligible to vote/hold office. That's what the book means when it talks about 'Franchise'. Most people are content with the freedoms civilians have and never even try to enter the service (I don't remember if the book ever gives a breakdown regarding what exact portion of the federation are 'citizens' vs. 'civilians', but I think it's implied to be single or low double digit percentages who are citizens and entitled to vote.)

Also, I don't think it's right to say they shrug off training deaths as 'nothing'. They had the whole regiment (I think that's the right term), including officers, spend 13 days looking for their bodies, had them buried with full military honors, and I believe (technically) posthumously granted them Citizenship, or at least actual ranks, not just 'Cadet such-and-such'.

I covered this in more detail in my top-level post on this thread, but Heinlen is trying to portray that value is (at least partially) on what something costs someone. The training is harsh, and even sometimes lethal, not only because it must be in order to be effective (or at least is made more effective by that risk), but also because the soldiers will value what they gain through it much more because of that risk.