Because for Biblical law the foundation is the one true God, the central offense is therefore treason to that God by idolatry. Every law-order has its concept of treason. Basic to the health of a society is the integrity of its foundation. To allow tampering with its foundation is to allow its total subversion. Biblical law can no more permit the propagation of idolatry than Marxism can permit counter-revolution, or monarchy a move to execute the king, or a republic an attempt to destroy the republic and create a dictatorship.
Heresy is punishable by death, if defined as idolatry.
And almost every act directed against Dr. Rushdoony's law-order is defined as idolatry and treason. This follows quite simply from his position that to not be under the Mosaic law is to have another god, and to therefore be in a state of idolatry. It does require a nation grounded in God's law-system to preserve that order by punishing the basic treason against it. Idolatry is thus not only punishable by law as socially detrimental, it is in fact a capital offense. It constitutes treason to the King or Sovereign, to Almighty God.