They considered it to be a temptation to idolatry. Thus it had to be destroyed. And Muhammad himself appears to be in favor of such actions: “Narrated ‘Aisha and ‘Abdullah bin ‘Abbas: When the last moment of the life of Allah’s Apostle came he started putting his ‘Khamisa’ on his face and when he felt hot and short of breath he took it off his face and said, ‘May Allah curse the Jews and Christians for they built the places of worship at the graves of their Prophets.’ The Prophet was warning (Muslims) of what those had done.” (Sahih Bukhari 1:8:427)
“Iraqis: Jihadis destroy ancient mosque in Mosul,” by Qassim Abdul-Zahra and Sameer N. Yacoub, Associated Press, July 24, 2014:
BAGHDAD (AP) — Islamic extremist militants blew up a revered Muslim shrine traditionally said to be the burial place of the Prophet Jonah in Iraq’s second-largest city, Mosul, on Thursday, residents of the city said.
The residents said the Islamic State militants, who overran Mosul in June and imposed their harsh interpretation of Islamic law on the city, first ordered everyone out of the Mosque of the Prophet Younis, or Jonah, then blew it up.
The mosque was built on an archaeological site dating back to 8th century BC and is said to be the burial place of the prophet, who in stories from both the Bible and Quran is swallowed by a whale.
It was renovated in the 1990’s under Iraq’s late dictator Saddam Hussein and until the recent militant blitz that engulfed Mosul, remained a popular destination for religious pilgrims from around the world.
Several nearby houses were also damaged by the blast, said the residents, speaking on condition of anonymity because they feared for their own safety.
The residents told The Associated Press that the militants claimed the mosque had become a place for apostasy, not prayer. The extremists also blew up another mosque nearby on Thursday, Imam Aoun Bin al-Hassan mosque, they said….