As Zero Hedge reports, Bosphorus is one of Turkey’s only trump cards when it comes to countering the Russians...Closing the strait would mean cutting one of Moscow’s key supply routes to Latakia (a principle port city of Syria), which would in turn force The Kremlin to either take a longer sea route or resupply the base by air.
Turkey is saying that Russia has violated the Montreux Convention, which according to Wikipedia, "is a 1936 agreement that gives Turkey control over the Bosphorus Straits and the Dardanelles and regulates the transit of naval warships. The Convention gives Turkey full control over the Straits and guarantees the free passage of civilian vessels in peacetime and restricts the passage of naval ships not belonging to Black Sea states. The terms of the convention have been the source of controversy over the years, most notably concerning the Soviet Union's military access to the Mediterranean Sea."
At any rate, Turkey seems to be reaching for excuses to close the straight here. While placing a serviceman on deck with a surface-to-air missile may not be the friendliest things to do, it's a far cry from setting up an airstrip in a sovereign country without permission, which is exactly what Turkey did last week.