"Except when justified by law" means that after the acceptance of Islam, if someone commits a crime which is punishable by Hadd this will be certainly imposed on him, or her, i.e., cutting of hand in case of theft, one hundred stripes or stoning to death in case of fornication, capital punishment in retaliation for murdering an innocent person. In this Hadith the words "it is for Allah to call them to account" means that if they are not sincere in the acceptance of Islam and would put up an appearance of Islam like hypocrites, or would commit a crime which is liable for Hadd but is somehow not detected by the Islamic court or authorities concerned, they will be taken to account for it by Allah, that is Allah will decide about them on the Day of Judgement. It is evident from this Hadith that so long as Kufr is present in this world, it is necessary to wage Jihad against it to finish it off, and so long as all the disbelievers do not openly accept Islam and adopt the Islamic way of life, Muslims are duty-bound to make Jihad against them.