For the same reasons israel gets free F-35s from the US. One reason is defense jobs for their respective defense workers. And the other reason is to test their equipment in combat, for marketing purposes. Imagine if an S300 were to shoot down an Israeli F-35. Or, conversely, if an F-35 destroys Syrian S300 launchers. In either case it would be a great marketing gain for the side that can declare "victory" and conversely, a great marketing loss for the side whose instrument has been bludgeoned. So as was the case with the Spanish Civil War of over 80 years ago, Syria has become a combat test bed for weapons on all sides. Of course the blood of the IAF pilots and the Syrian S300 operators will be spilled in these unfortunate duels.