Because Johnson, irrespective of what he had done last time, had a right to expect that someone would not come on the field seeking to deliberately injure him.
Warnock had every right to play Santos. But Santos said he was going to get Johnson. Warnock said he was aware of that. This was all before the game. Warnock put Santos on the field knowing he intended to commit an assault. I think that was a reckless decision that deserves condemnation.
I don't know about you, but if I am a manager and a player tells me that if I pick him for a particular game he intends to deliberately injure one of the other players, I don't let him take the field.
Ironically of course, one of the things covered in Warnock's book is the allegation that he has told players to deliberately injure opposition players on previous occasions. Warnock denies this, and then weakens his denial by referring to a number of occasions where he has been accused of this. Some of these accusers are the Stan Ternants of this world but others are people with no axe to grind against Warnock (Steve Bull is one).