Syria is allied with Hezbollah, and the declared goal of Hezbollah is to destroy Israel. Therefore, any arms acquired by Syria can either be transferred to Hezbollah or fall into their hands if and when the Assad regime falls. Moreover, some of these weapons may fall into the hands of the jihadists.
10 arrested at Israeli Arab protest in Ramle (Haaretz)
from the article: Report: Russia's S-300 missiles may not reach Syria until 2014