The facts are that no matter what kind of deal you signed to enlist, once you got out of active duty back then nobody would force you into a reserve unit.
Think about it ~ here you have brave men who've volunteered to go into the armed forces, or they've been drafted and showed up at the induction station rather than Canada ~ now, they are finished with the hard part of being in the military and somebody wants them to go to a drill once a month (or so) where most of the people will have successfully evaded the draft or not enlisted.
And they have real guns and bullets handy!
It was a normal practice to excuse folks who'd completed their active duty (2 years or more) from any reserve duty.
End of story.
Rules be damned.
Kept the reserves pure and away from the real soldiers. Only a madman would have thought it useful to mix them up.
Kerry was, for better or for worse, on the right side of the rules in this instance.