They don't call Hamas terrorists because the word is emotional and tendentious. They don't call the IDF terrorists for the exact same reason. The BBC's remit is to try to be neutral. Tabloids and politicians, on the other hand, are free to use that expression at will. If you want the Beeb to call Hamas terrorists, then I challenge you to tell me why you don't think that the Beeb shouldn't also call the IDF terrorists. After all, 18000 innocents dead at their hands, including 8000 children? Unless you think that's OK?   

The old saw is that the terrorist is the man with the small bomb. There's something in that.