The reason why Muslims are loath to criticize other Muslims is because of the Quran and Hadiths which tell them that it is wrong for a Muslim to side with a disbeliever against another Muslim.
Abu Musa reported Allah’s Messenger (may peace be upon him) as saying: A believer is like a brick for another believer, the one supporting the other. Sahih Muslim Book 032, Number 6257
Narrated 'Abdullah bin Umar: Allah’s Apostle said, “A Muslim is a brother of another Muslim, so he should not oppress him, nor should he hand him over to an oppressor. Whoever fulfilled the needs of his brother, Allah will fulfill his needs; whoever brought his (Muslim) brother out of a discomfort, Allah will bring him out of the discomforts of the Day of Resurrection, and whoever screened a Muslim, Allah will screen him on the Day of Resurrection.” Volume 3, Book 43, Number 622
Narrated 'Abdullah bin 'Umar: Allah’s Apostle said, “A Muslim is a brother of another Muslim. So he should neither oppress him nor hand him over to an oppressor. And whoever fulfilled the needs of his brother, Allah will fulfill his needs.” Volume 9, Book 85, Number 83
Narrated 'Abdullah: Allah’s Apostle said, “Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief).” Volume 8, Book 73, Number 70
Narrated Anas bin Malik: Allah’s Apostle said, “Help your brother, whether he is an oppressor or he is an oppressed one.” Volume 3, Book 43, Number 623
Narrated Anas: Allah’s Apostle said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, “O Allah’s Apostle! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?” The Prophet said, “By preventing him from oppressing others.” Volume 3, Book 43, Number 624
Narrated Sahl bin Sad As-Sa’di : Allah’s Apostle said, “To guard Muslims from infidels in Allah’s Cause for one day is better than the world and whatever is on its surface, and a place in Paradise as small as that occupied by the whip of one of you is better than the world and whatever is on its surface; and a morning’s or an evening’s journey which a slave (person) travels in Allah’s Cause is better than the world and whatever is on its surface.” Volume 4, Book 52, Number 142
The Unbelievers are protectors, one of another: Unless ye do this, (protect each other), there would be tumult and oppression on earth, and great mischief. Sura 8:73
Verily, this brotherhood of yours is a single brotherhood
The Believers are but a single Brotherhood.