The strategy representation could also be rationalized as the Mars actively positioning their forces near most efficient transfer routes between points of inter

The strategy representation could also be rationalized as the Mars actively positioning their forces near most efficient transfer routes between points of interest. There seems to be relatively few places in the Belt and outside where anyone would want to go to, and guarding empty space where you'd expect no one to pass wouldn't make much sense. So ships transiting from Earth to the Belt would be going through Mars' solar orbit doughnut-of-influence. This of course ignores that with the Epstein drive magic, one could always trade travel time for going around ie. off-plane. There's plenty of, well, space for that. (Edit: according to the no-stealth-in-space link in a sibling post, one would be seen doing that, and likely get chased for it as being suspicious).