Both land and naval units that engage in combat remain in the area of combat and cannot be moved in the non-combat phase. Retreated units have to stay in the territory/sea zone they were retreated to. The only units that can move after Combat (and in fact HAVE to move in almost all cases) are aircraft.
If you need a basis in reality for this rule, consider those forces to be being re-supplied, repared, reinforced, replacing casualties, etc. as well as securing the area, reparing damaged infrastructure (bridges, airfields, etc) during the non-combat phase. For retreated units, consider them to be regrouping, replacing lost equipment, etc.
