The leader of Syria is a Sunni terrorist but he stays in place because he blocks arms shipments through Syria from Iran to Hezbollah. That suits Israel. Israel doesn't care how the Syrian leader treats his own people so long as he continues to disallow arms to Hezbollah from Iran. And the USA goes along with whatever Israel wants.
It has nothing to do with how the Iranian regime treats its own people. Just look at how the Saudi regime treats its people.