[The Real Slim Shady]

It's simply a matter of Terminology

Anyone that isn't Pro-Assad is a 'rebel'. This means ISIS, FSA, and whoever else is involved. Russia is there to crack down on rebels. They are Pro-Assad. They have been providing Assad weapons and shit since the start of the conflict.

The US is there to rid Syria of it's Government, It's backing a subset of the rebels trying to rid Syria of Assad. Only problem is that they disagree with ISIS as well - so they're now fighting some rebels and the government... making the whole thing a big cluster fuck that they ultimately cannot "win". Russia can potentially "win" by successfully putting Assad back into complete control of the Syrian region. There is a clear combatant and a clear winner. The US doesn't know who they want in control, but they don't want Assad and they don't want ISIS.

This is just another proxy conflict between the US and Russia to add to the very long list:

Chinese Civil War
Iran Civil War
Korean War
Cuban Revolution
Vietnam War

Are just the major ones you've probably heard of. Theres a much longer list of smaller conflicts.