Arguments that in other languages other countries call as they want here go by. We are talking about Russian language. And officially in Russian there is the word "Belarus" and not "Belarus" (I write now and even mozilla the second version underlines red).

It is correct to emphasize that there is no connecting letter A in Russian either. But there is a connector O: Belorussia, Montenegro, Krasnogorsk. White Russia, Black Mountain and Red Mountain.