That's reality. Obama and Kerry obviously recognize that. (With US-backed dictators the only way the world can put pressure on them is by negotiating with the US.)
It is in Russia's national interest to maintain their only naval base in the Mediterranean. It is in Assad's national interest to have a generous arms suppler, an effective foreign air force to fight for him and a country on the Security Council to protect it from UN initiatives.
Tunisia, Libya and Egypt had no Russian military bases and it was not in Russia's national interest to get involved when their dictators were challenged. Ukraine did have a large Russian military base. When the government in Kiev was no longer friendly, the province the base is in was no longer part of Ukraine. Military bases mean a lot to Russia.
Assad is no dummy. He knows that the Russian naval base is his ace-in-the-hole. As long as Russia perceives that his continued rule is the best way to preserve their access to the base, they will do what they can to protect him. The US did the same in the Philippines with the Marcos dictatorship. Marcos portrayed himself as the guarantor of US access to its huge air base and naval base in the Philippines. The US supported him for many years and supplied arms to him in his fight with 'communist rebels' and Muslim insurgents.
Assad's main worry should be that one day Russia may perceive a negotiated settlement that includes guarantees about its access to the base as preferable to continuing to back him personally. I am sure he gets a little nervous when Russia engages in negotiations that affect his future.