What would you expect the habitats to have instead of nitrogen? You wouldn't want a high oxygen atmosphere. Not only is it an extreme fire risk (see Apollo 1), but humans aren't built for it - oxygen toxicity exists.
Those are both hazards from having too much absolute oxygen, not too much relative oxygen. Our current launch craft use an atmospheric mix so they are safe on the launchpad, and the ISS uses an atmospheric mix to easily interface with those launch craft. On mars you don't need to worry about space shuttles coming to visit, so .25atm of mostly-oxygen would be fine.