Yeah this is really dumb. Apparently the reason for forcing non muslim to convert upon marrying muslim is due to the sultan potentially(very likely) marrying non muslims, and if they seperate in the future, the wife can bring the divorce into international court and have custody of the offspring. The muslims want to ensure that no consequences arise from that such as the non muslim raised children come back and claim their fair share of throne. Eventually this law evolves to be enforced on citizens. This explains why the muslim populations are growing like rabbit in Malaysia.