In East Malaysia, is the Christian God referred to as Allah? Yes. Do they speak in Bahasa Malaysia? Yes. Is the Christian God referred to as 'Allah' in Bahasa Malaysia? Yes. So is it not wrong? No (in East Malaysia). Is Bahasa Malaysia the same throughout the whole of Malaysia? Yes. Do we all sit for same exams? Yes.
So why can't Allah be used in West Malaysia as well? Because we say so. And because there are enactments. Can you quote anything from the Quran that prohibits the use of Allah by Christians? No.
So why do you say it is prohibited for us to use? For the sake of harmony. Isn't it better to educate the people that it is okay for non-Muslims to use the word? No. Why? Because we say so. Who is this we?