Because Britain ruled the Indian sub-continent for 300 years, from the Persian Gulf to Burma, and the whole region is part of Asia. To us, Asians are what they call Orientals. It's just how they phrase it.
I do understand all that - what I don't understand is the aversion to using the word "Muslim" when that's what we are talking about.
If Christians in the Philippines were rioting - what would the Philippine press call them - "Asians" or "Europeans"? Of course not - they would call them "Christians".
Same principle.