Well, the official line is Russia is Arming Assad, because they believe (at least officially) that he's the best wall of defence against ISIS. Now, some, but not all of the rebel forces are actually aligned with ISIS. Other, we assume, are simply freedom-loving people, who may or may not join ISIS at a later date.
Guess who the U.S. is arming and why we're annoyed at Russia and don't want their planes flying into Syria?
Some of the U.S. arms may end up in ISIS hands, but rest assured: Sales to the Middle East in U.S. arms ensure a nice wall of weapons all over the place. It's the American way. And if you had stock in Lockheed Martin or Boeing, Airbus, etc... in the last decade, decade-and-a-half, you made a lot of dough.
Of course, there is the small matter of Assad being a monster. But diplomats are now worried about a thing they call stability.
Which is why major wars in the Middle East are probably never a good idea.