Not all Syrian rebel groups are "terrorists". Indeed, most do not commit terrorist acts, however you might define the latter. Assad otoh has one of the most abysmal human rights records in the world.
Every Middle East regional power backs a faction or eight in Syria. Israel does the same. One man's rebel is another man's terrorist is another man's freedom fighter.
Of course, Putin's playing piece is Assad and they will both tell you all day long that all Syrian "rebels" are "terrorists" and that Assad is the nicest guy in the Middle East.