It's rather because Assad is pro-Hezbollah and pro-Iran, as well as anti-Saudis and other US allies. Russian presence in this case was certainly annoying US but the US positions in Middle East don't revolve around Russia.LOL You think they want to get rid of Assad on ethical grounds? They want to get rid of Assad because Assad is pro-Russian.
The United States and its allies deliberately worked up sectarian strife as a means to back their colonial attacks, and meanwhile heavily support the Zionist child-murderers who hate all 'Arabs' on racist grounds. Naturally those of a different religious persuasion from the Saudi-backed madmen of ISIS get together to defend themselves against killers of both kinds.