It depends on which "restaurants" you mean. If you mean the wonderful chef-run restaurant downtown that cooks everything from scratch then you have a very competent cook staff that know how to break down a chicken, then butterfuly the breast, season it perfectly and cook it so that it is tender and juicy.
But if you mean Applebuds, Chilly's, or Pizzeria Una's chain restaurants there is something else entirely going on. David Kessler is the author of "The End of Overeating," a book that describes the food industry's process of making our food more palatable. Here I quote "In many cases, the preparation of the dish begins in the factory, where the meat is processed, battered, fried and frozen. Like many processed meats, chicken can contain as much as 19% of a water/sugar based solution, and salt is added as well. More salt and other spices are added before the battered chicken is prebrowned in oil and frozen. At the restaurants, the meat is deep-fried in oil again before it's served."
Not all chicken is deep fried for example but it is at least injected with a solution of salt/sugar/fat at the very least if not more.