The question isn't whether the Pope must apologize to Muslims. The question is, who doesn't? Sheikh Qaradawi, leader of the Muslim Brotherhood, issued the fatwa.
International Union Of Muslim Scholars: The Pope Must Apologize To The Muslims MEMRI
On September 13, 2012, the day before Pope Benedict XVI began his visit to Lebanon, the International Union of Muslim Scholars (IUMS), headed by Sheikh Yousuf Al-Qaradhawi, issued a communiqué attacking him. The IUMS called on the pope to apologize for the acts of slaughter committed by the Crusaders in Spain, and for a 2006 lecture in which he claimed that the Muslim religion supports violence.
The following are excerpts from the communique:
The Pope Should Apologize To Muslims "As He Apologized To The Jews"
"While the [International] Union [of Muslim Scholars] is working to calm the rage of Muslims worldwide, [which was aroused] by non-Muslims insulting the honorable Messenger [i.e. the Prophet Muhammad], [so that they] confine themselves to peaceful protests and not attack any embassy or Christian site – it [also] calls on the pope to apologize to Muslims for his lecture [in 2006 in Regensburg, Germany], his apostolic exhortation, and the slaughters committed by the Crusaders [against Muslims] in Andalusia, just as he apologized to the Jews.
"It is known that the IUMS espouses the mainstream [Islamic] ideology, which is based on peaceful coexistence and respecting the rights of others, and calls for constructive dialogue as the best way to solve problems and build balanced relationships. Therefore, the IUMS tried to conduct dialogue with the Vatican, but felt that it was one-sided. In addition, the current pope, in his famous lecture, made false accusations about Islam and its magnificent Messenger, may Allah's prayer and peace be upon Him. The IUMS demanded that he apologize for the content [of the lecture], but he did not do so. As a result, the IUMS ceased dialogue with the Vatican until [the pope] apologized or until the situation changed...
"During his visit to Lebanon, the pope means to sign the apostolic exhortation, which includes a series of dangerous ideas and messages, among them a clear call to Christianize the world, a warning against the Islamicization of Muslim society; and [attempts to] frighten Christians about political Islam in the region. This helps to spark the fire of fitna among partners in the [Lebanese] homeland (Muslims and their Christian brethren), especially in light of attempts by some Christians to distribute a movie insulting the magnificent Messenger, may Allah's prayer and peace by upon Him, and attempts by one American pastor to burn the noble Koran, and to set a trial date for the magnificent Messenger..."
The Pope Warns Against Political Islam, But Is Engaged In Political Christianity
"It is strange that at a time when the pope is warning against political Islam, he himself is engaged in political Christianity in the fullest sense of the term. In light of what was mentioned, the IUMS stresses the following:
"1. The IUMS reiterates its call to the pope to apologize for his famous lecture at a German university and for the acts of the Crusaders in Andalusia and Al-Sham [greater Syria] during the Crusades, which included murder, expulsion, arson and tyranny, just as he personally apologized to the Jews for Christian acts against them.
"2. The IUMS calls on the pope not to ratify the document known as the apostolic exhortation and not to sign it, since it contains erroneous information and incites sectarian fitna between Muslims and Christians who have lived as partners in the homeland, as evinced by the leaders of all Islamic movements stressing the full rights of Christians.
"3. The IUMS demands that Islamic countries and peoples fulfill their duty to their ummah and their faith by applying political and economic pressure in order to defend Islam and weak minorities.
"4. The IUMS demands that Muslim scholars fulfill their duties of guidance and explaining the truth."