Actually, airlines in the USA are generally running almost all their flights at full capacity right now. Especially the major carriers. Competition has made them run with nearly 100% load factors nearly every day of the week.
Business travelers dominate Sunday, Monday, Friday and Saturday leaving Tuesday - Thursday for non-business travelers. Middle of the week is where the deals are now, but because they have reduced capacity so much, business travel is being squished into these days as well.
The 2015 domestic load factor is 83.68% right now - which is indeed extremely high. Many flights are full at such a high load factor, but not all of them are. There are still "red-eyes" and other off-peak flights that have loads well below 50%, and the fact that they don't charge high fares (to cover the full cost of flying the plane) for those flights - the rates are discounted in fact - shows that the principle of some flights subsidizing others is still in place.