What assumptions about the importance of individual rights and civic duty lie behind that decision?

The assumption that military service is a civic duty, for one. America is one of the only countries that does not require (or at least encourage) military service. It was viewed, especially in 1913, as a necessary means of protecting the country, and one that may require involuntary service.

Additionally, the 13th amendment was never written with the military draft in mind, the assumption about individual rights is that they are rights to be free from private action that is unreasonable, not necessarily from reasonable government action (like instituting a draft to protect the country).