Preheat oven to 450°F. Rub pork all over with oil. Mix rosemary, ¾ teaspoon salt, and ½ teaspoon pepper in small bowl. Rub all over pork.
Place pork, bone side down, in large flameproof roasting pan. Roast 15 minutes. Reduce oven temperature to 350°F. Roast 15 minutes more. Add sweet potatoes and pears to pan, stir gently to coat with pan juices, and spread as evenly as possible around roast. Season with salt and pepper. Roast, occasionally stirring sweet potato mixture, until instant-read food thermometer inserted in center of roast reads 145°F and the potatoes and pears are tender, about 1 hour more. Transfer pork to carving board. Transfer sweet potato mixture to ovenproof bowl, tent with aluminum foil, and keep warm in turned-off oven. (If needed, increase oven temperature to 400°F, and continue roasting sweet potato mixture until tender, up to 15 minutes longer.) Let pork stand for 10 to 15 minutes.
Heat roasting pan over high heat. Add cider, bring to a boil, scraping up browned bits in pan with wooden spoon, and boil until reduced to ¾ cup, about 5 minutes. Pour into sauceboat.
Carve pork. Transfer to serving platter, surround with sweet potato mixture, drizzle with 3 tablespoons of cider sauce, and sprinkle with rosemary. Serve with remaining sauce on the side.
*Tip: If you desire to tie the roast, cotton kitchen twine (also called butcher's twine) is handy to have on hand for a number of chores, especially tying roasts to help them keep their shape during cooking. If you don't have it, use unwaxed and unflavored dental floss. **Substitute 1 cup apple juice and 1/2 cup dry vermouth for the hard cider, or use all apple juice.