Re: why !!! give one good reason
This is from al-islam.org
Muawiyah not only fought Imam Ali, he cursed Imam Ali as well. Furthermore,he did force/make everybody to curse Ali (AS). To prove it, we begin withSahih Muslim: Narrated Sa'd Ibn Abi Waqqas:
Muawiyah, the son of Abu Sufyan, give order to Sa'd, and told him: "What prevents you that you are refraining from cursing Abu Turab (nickname of Ali)?" Sa'd replied: "Don't you remember that the Prophet said three things about (the virtue of) Ali? So I will never curse Ali."
Sunni reference: Sahih Muslim, Chapter of Virtues of Companions, Section of Virtues of Ali, Arabic, v4, p1871, Tradition #32.
For the English version of Sahih Muslim, see Chapter CMXCVI, p1284,Tradition #5916
Below is more references in Sahih Muslim about Sunnah cursing Imam Ali(AS), to prove that people were urged/forced to curse Ali in public,otherwise they would face a costly sentence. It is narrated on theauthority of Abu Hazim that:
The Governor of Medina who was one of the members of the house of Marwan called Sahl Ibn Sa'd, and ordered him to curse Ali. But Sahl refused to do so. The governor said: "If you don't want to curse Ali, just say God curse Abu Turab (the nickname of Ali)." Sahl said: "Ali did not like any name for himself better than Abu Turab, and Ali used to become very happy when somebody would call him Abu Turab."
Sunni reference: Sahih Muslim, Chapter of Virtues of Companions, Section of Virtues of Ali, Arabic version, v4, p1874, Tradition #38.
One of the ugliest innovations that started during the reign of Muawiyahwas that Muawiyah himself, and through his order to his Governors, theyused to insult Imam Ali (AS) during the Sermons in the Mosques. This waseven done on the pulpit of the mosque of the Prophet in Medina in front ofthe grave of the Prophet Muhammad (May Allah bless him and his Progeny),so that even the dearest Companions of the Prophet (PBUH&HF), and Imam Ali(AS), his family and his near relatives used to hear these swears withtheir ears.
Sunni references :- History of al-Tabari, v4, p188- History of Ibn Kathir, v3, p234; v4, p154- al-Bidayah wa al-Nihayah, v8, p259; v9, p80
On insulting Ali Ibn Abi Talib and cursing him during the Umayah periodstarting in Muawiyah's reign, it is reported that: "Ali Ibn Abi Talib (ra) was cursed on the pulpits (manabir) of the east and west...", during the time of Muawiyah.
Sunni reference: Mu'jam al-Buldan, al-Hamawi, v5, p38
In her letter, Umm Salama, the wife of the Prophet (PBUH&HF) wrote to Muawiyah: "...You are cursing Allah and His messenger on your minbar, and that is because you are cursing Ali Ibn Abi Talib and whomever loves him, and I am a witness that Allah and His messengerloved him." But no one paid any attention to what she said.
Sunni reference: al-Aqd al-Fareed, v2, p300
Add to this the martyrdom of Hujr ibn Adi(RA) pious companion of Prophet(saw) who was killed simply for refusing to curse Ali(as)so much so that Ayesha(r) and Abdullah ibn Umar(ra)even criticized Muawiyah for this act
Let me add if you think they are all shia writers in taqqiyah in think whatever the hell you like but the fact **remains that these events are pretty wellrecorded in history**
Almost all western authors on this subject have also recorded this