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: