This is a parabolic vision and must be interpreted as such. In it God is represented as doing that which He does not restrain. God does not coerce the will. When evil men willfully choose to follow deception He does not intervene.

 Since God is supreme, His refusal to restrain the forces of evil is often represented as though He directly sends the evil. An example may be found in the incident of the fiery serpents (Num. 21:4-9). According to the narrative as related by Moses, “the Lord sent fiery serpents among the people” (Num. 21:6). However, these “fiery serpents” were not suddenly created or miraculously transported from some other region for the occasion; they already infested the wilderness area through which the children of Israel were traveling and would have been a source of real danger and the cause of frequent deaths had not God, by miracle, subdued these venomous reptiles. But when the people turned against the God who protected them from the many hazards of the desert, God simply withdrew His protection, and death was the result (see PP 429). Thus it was in the case of Ahab. Satan was already at work through the agency of the false prophets, and God simply did not prevent the course the king had chosen for himself.