This may be explained as follows:
Many leading scholars have explicitly declared that someone who performs an act in a form whose ruling is disputed is cursed. Among them is ʿAbd Allāh ibn ʿUmar, may Allah be pleased with them both. He was asked about a man who married a woman to make her lawful again for her former husband, without either the woman or her former husband knowing of his intention. He replied: “This is fornication, not marriage. ‘Allah has cursed the man who marries a woman to make her lawful again for her former husband, and the former husband for whom he does so.’”
This is reliably reported from him through several chains of transmission, and similar statements are reported from others, including Imam Ahmad ibn Hanbal, who said: “If he intends to make her lawful again for her former husband, then he is marrying for that purpose and is cursed.” Such statements are transmitted from numerous leading scholars concerning many disputed cases involving wine, usury, and other matters.
If the curse pronounced in the revealed texts, and the other threatened punishments reported in them, applied only to cases on which there is agreement, then these scholars would have cursed people whom it is impermissible to curse. They would therefore incur the threatened punishment mentioned in several hadiths, such as the Prophet’s saying, may Allah bless him and grant him peace:
“Cursing a Muslim is like killing him.”
And his saying, may Allah bless him and grant him peace, as narrated by Ibn Masʿūd, may Allah be pleased with him:
“Reviling a Muslim is wickedness, and fighting him is disbelief.”
Both are reported by al-Bukhārī and Muslim.
Abū al-Dardāʾ, may Allah be pleased with him, reported that he heard the Messenger of Allah, may Allah bless him and grant him peace, say:
“Those who habitually disparage and curse others will be neither intercessors nor witnesses on the Day of Resurrection.”
Abū Hurayrah, may Allah be pleased with him, reported that the Messenger of Allah, may Allah bless him and grant him peace, said:
“It does not befit a person of utmost truthfulness to be given to cursing.”
Both are reported by Muslim. ʿAbd Allāh ibn Masʿūd, may Allah be pleased with him, reported that the Messenger of Allah, may Allah bless him and grant him peace, said:
“A believer is not one who habitually disparages or curses others, nor is he obscene or foul-mouthed.”
Reported by al-Tirmidhī