Because it serves US foreign policy goals, which in turn reflects the combined goals of various interest groups such as the organized Miami Cuban exile community. The latter explains why good terrorists include such people as Orlando Bosch.
Of course, any discussion of terrorism requires a definition: The US Joint Chiefs of Staff defines it as "The unlawful use or threatened use of force or violence against people or property to coerce or intimidate governments or societies, often to achieve political, religious, or ideological objectives." Under this definition, Bush, Rumsfeld, _et al_ are terrorists.
-- Jim Devine