In World War Two, the Allies (namely the United States and the United Kingdom) made a pact with the Soviet Union and its leader Joseph Stalin to combat Adolf Hitler and Nazi Germany. Initially, Hitler had made a non-aggression pact with Stalin as they both carved up Poland between them, but when Operation Barbarosa occured (the code name for the invasion of the Soviet Union), Stalin made common cause with the west. Although the west knew Stalin to be an evil dictator, just as brutal as Hitler, it was a question of 'deal with Hitler and worry about Stalin later'.
It seems this is the case in the Middle East today. The coalition nations, with the US leading and the UK and other nations following behind (including other Arab states), bomb Islamic State targets round the clock. The Islamic State are an enemy of Assad of Syria and so it seems the coalition nations, just as in World War Two, have made common course with the Assad regime of Syria..
Unofficially of course, Syria is not part of the coalition nations attacking Islamic State. However, coalition planes are flying in Syrian airspace, just as Syrian planes are flying to attack Islamic State and other targets. This leads to the assertion that the Syrian and coalition air forces are not engaging each other, therefore there must be some unofficial agreement that follows, that the allies and Syrians fighting a common enemy in IS so do not engage each other in combat.
The coalition recognize that Assad is a brutal dictator, but have shelved any plans to take action against him. The West may realize therefore, that the Alawite regime in Damascus under President Assad is holding the line against Islamic State and other Al Qaeda inspired groups.
Add into the equation that Iran, just like Syria, is not part of the official coalition against IS. However, it must have the tacit permission of the coalition nations to attack Islamic State, on the ground or via the air, whilst not clashing with coalition aircraft. It also follows that Assad happens to be a friend of the Iranians. .
Assad's territory in Syria has been much reduced, with rebel groups who oppose him like the Free Syrian Army. Most of the groups are Al-Qaeda inspired organisations, and are off shoots of Al-Qaeda like the Islamic State. However, Assad still has a formidable air force and army and is still able to attack and defend against any incursions from any rebel group.
The United Nations have stated that should any peace deal come to the fore and be hammered out at the negotiation table Assad (dictator or not) must be involved in any final agreement that ends the fighting.