The United States of America is supposed to be fundamentally about its Bill of Rights.
It's real founding document is its Declaration of Independence. It's Constitution required amending to include the Bill of Rights before that Constitution was acceptable to the People.
Perhaps the most central tenet in that Bill of Rights is the first part of the First Amendment, which part enshrines as much freedom of religion as the Founders were able to word: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof...."
Sharia stands exactly opposed to that. When the US government supports those who violently fight for the establishment of sharia anywhere, that government is serving to undermine the very essence of its own foundation.
What is sharia? It is Islamic theocracy. It is the position that Islam is the one and only law. With sharia, there is no secular government. There is only rule by Muslim clerics.
Right now, Russia has been pressing the "West" to designate "Ahrar al-Sham" as a terrorist group.
The Obama administration has refused. What is Ahrar al-Sham?
The contents of this link are a bit dated, but you should get a decent background understanding of the group by reading it: Ahrar al-Sham.
Why has Russia ask for Ahrar al-Sham to be designated a terrorist group? Syrian villagers describe massacre by militant group spared from UN terror blacklist (EXCLUSIVE).
This is important to understand:
The Alawites - followers of an offshoot of Shia Islam who are mainly concentrated in the Mediterranean coastal provinces of Latakia and Tartus - are considered heretics by some Sunni Muslims. (Source: Alawite civilians 'killed in homes' in opposition attack on Hama village)
Ahrar al-Sham members oppose the Alawites because of the Alawites' theology. Alawites would not be allowed to practice their religion under Ahrar al-Sham's sharia.
You need to also understand that President Assad of Syria is the leader of a secular government. It hasn't had a perfect track record on freedom of religion, but none of the significant powers fighting against Assad on the ground would be better in that regard, not even close.