Christian Arabs use the word because it has been that way since.
Christian Indonesians and Christian Malaysians use or try to use the word to gain advantage in their Proselytization of Muslims in this part of the world.
About the use of the word, someone just smelled how slick, how sly, how savvy, how shrewd, and how cunning these people are - I am just trying to put the perspective before you in limited words. Please be game.
Some group are trying to take advantage of the word, but it is the Malay group of the last decade. The word has been used by non-chinese christians for centuries without any issue - until it suddenly became an issue:
"Christians in Indonesia and Malaysia also use ‘Allah’ to refer to God in their languages. This comes through traditional translations of the Bible made back in the 16th-17th centuries which put the Hebrew’s ‘Elohim’(‘God’ in English holy books) as ‘Allah’.
But in 2007 the central Malaysian government decided to put an end to the old tradition and limited the usage of the term to the Muslim context only." http://rt.com/news/malaysia-allah-forbid-musl...
Just a background before you proceed further, readers:
Christian Indonesians and Christian Malaysians comprised of mostly citizens of Chinese ancestry, who have been in competition with Muslim natives in economic and political sense.
Chinese christians in Malaysia don't use the word Allah...(I'd turn away from a church if it suddenly use Allah to refer to god). Only the non-chinese christian group that would have no issue with the word that they already use it as part of their tradition - when today's Malays try to claim exclusive right to that word, they are simply robbing the other group's tradition.... but what don't we already know about certain Malay's behaviour? Ibrahim Ali for one? Are you part of his Perkasa group that would call for holy war against christians? Howcome I don't see much complains about people like him as: "someone just smelled how slick, how sly, how savvy, how shrewd, and how cunning these people are" - the language you applied to the group who simply tried to preserve their centuries old tradition of using the word Allah?