Not all people know this. Some ate food for fake gods before. When they eat now, they still think it is for fake gods. Their hearts feel weak and bad. ️
But not everyone has this knowledge. Some people are still so accustomed to idols that they eat such food as if it were sacrificed to an idol. And since their conscience is weak, it is defiled.
KJV
Howbeit [there is] not in every man that knowledge: for some with conscience of the idol unto this hour eat [it] as a thing offered unto an idol; and their conscience being weak is defiled.
ASV
Howbeit there is not in all men that knowledge: but some, being used until now to the idol, eat as of a thing sacrificed to an idol; and their conscience being weak is defiled.