Not all shuttles are the same. Delta and US Airways Shuttles are to fly businessmen between the cities of the Golden Triangle (Washington, Boston, and New York). Financiers, lawyers, lobbyists, etc. need to go to another city in the triangle for a day, often on little notice.
United Shuttle is a whole nother thing. Their purpose is to compete with Southwest and other lowfare airlines in the west, like Metrojet and Delta Express do in the east.