The U.S. Army and U.S. Marines have different doctrines. How the Marine Corps has been used as an occupying force, which is standard for the U.S. Army, since the beginning of GWOT was the decision of our government, not by doctrine.
The Army is not big enough to handle their traditional mission by themselves. It will always require the Marine Corps as well. Plus, the services will always argue to include their units in every commitment worldwide.