The only thing that is surprising is that anyone is surprised at this. They are doing what they always do: make money and increase their influence while pretending to hew to the international consensus. Ditto, BTW, for China. It should also be remembered that the Russians consider Iran a natural client, vying throughout the 19th Century with the British for effective control there.
But all should remember that the Russians, Chinese, and, in particular, the French were taking bribes and selling arms to Saddam Hussein throughout the time that U.N. sanctions were in place. Why should Iran be any different?