The servants were not aware that the householder had an enemy. Many people who believe in the Lord do not believe in Satan. Such are ignorant of his designs and do not recognize that some people are working for him. This ignorance enables the enemy to plant even more tares.
What do we do with tares? -> We are not to destroy the tares because it makes us guilty of murder undermining our own lives. -> We are not to fertilize them enabling growth and reproduction -> We are to preach Christ to them, to reprove them, rebuke them and exhort them 2Ti 4:2