What's confusing about them is that the way it's written, it's the "offense" being punished, not the person. I think it should be: "His offense was not found too serious to be punishable." "His offense was not found serious enough to be punishable." more simply: "It was not serious enough to be a punishable offense." or: "His offense was not found too serious for him to be punished." "His offense was not found serious enough for him to be punished."

I think the top is the correct. Because I think enough is use for the acitve not passive form.