hmm, don't think it's that easy. don't think there's an actualy rule
for instance you say "the balcan" (albania - kroatia - servia - some more), but at the same time, they say "scandinavia" (norway - sweden - finland) without "the"
Scandinavia is a geological definition of a certain area, and therefore, not plural. "The Balkan" I have never heard; I've always heard "The Balkans" which refers to the Balkan states. Perhaps this exception comes out of that particular diction. To me "the Balkan" once again refers to the actual land area, not the states that occupy it, thus making the usage singular.