Edit: I've found
more info on it, it seems common in Shafi'i and Hanbali communities especially.
They're not really "communities" but schools of Sunni Islamic law. Shafi'i and Hanbali law requires FGM be performed, whereas the other two (probably listed on that page too I assume) 'only' consider FGM to be an 'honorable' action to be encouraged. Sunni Islamic Law is developed from the collected Hadith traditions, which do mention (and support) FGM. This stuff carries a lot of weight for Sunni Muslims.
Shia Islam follows a different school of legal tradition, but still has issues with FGM support and acceptance. There are (as you mentioned) modernist scholars, both Sunni and Shia, who've pushed back against FGM, but they've encountered their own pushback from the orthodoxy who support it.
IMO we should support reformist Muslims who are right now working on driving down FGM. But it's really a notoriously difficult problem to eradicate; Sunni Muslims who want to hold onto orthodoxy find a lot of support for FGM within the Hadith collections.
Nice avatar btw.
Ali AlSistani, a respected Shia cleric, said: "If the purpose of female genital circumcision is cutting clitoris this operation is not right and is not a religious tradition. If the girl is hurt, it is prohibited. Female genital (sexual) mutilation or cutting off a part of her genital is certainly a crime against girls and there is no permission and justification for parents to do this operation."
Again, Islam is broad. The problem of FGM is heavily concentrated within Sunni Muslim jurisprudence; most of these people would call al-Sistani an apostate, hypocrite, etc, and consequentially his opinion as a Shia isn't relevant to them, because the Sunnis have their own scholars and their own schools of law. This is why it's a really big bitch of a problem. Also, for every Shia cleric who says no on FGM, someone else with equal or greater clout (like the Ayatollah) has said yes.