Like the classical congenital myopathies, the muscular dystrophies are genetically determined diseases of skeletal muscle.  Unlike the classical myopathies, there are not unique pathological findings, like nemaline rods or central cores.  So historically, these disorders have been placed in a separate category.  However, they are a subset of congenital myopathies.

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.