You are taking your battle action and have just declared your attackers. Your opponent now plays an action that corrupts one of your attacking units? What happens? Is that unit removed from the attack or since it was already declared as an attacker does the corruption have no affect now? Obviously your opponent could have played the corruption action after you declared the zone you were attacking, but before you declare you attackers. This could be an issue though if your opponent was trying to see what forces you commit to the attack before he/she chooses which unit to corrupt.
Corruption only takes away the player's ability to declare the said unit as attacker or defender, it does not however, remove the unit from a fight once the unit has joined battle.
I'm not sure if an AGoT reference will help you understand this issue better, but if it does: I remember seeing a similar situation in AGoT where removing a relevant challenge icon from a character that already had engaged in the challenge does not remove the character from the challenge. The presence of one of three challenge icons allows the said character to be declared as attacker or defender and participate in the corresponding challenge. You can see this particular parallel (among many) in the two systems designed by the same designer.