Just curious. Can spells that bring creatures from another plane to the Prime bring a Petitioner? All sorts of other planar natives are vulnerable to being pulled from the Inner or Outer planes and brought to the Prime (or some other plane), so should Petitioners also be summonable? If not, why?
(I got thinking along these lines when I saw that the new DMG mentioned Dwarves with the Celestial type, and thought they would be a nice addition to the few Celestials available for summoning in 5e. Then I realized that they were probably meant to be Petitioners... hence the question.)