Arab is not a reference to spiritual beliefs. It is more of a cultural and linguistic term. You can be fully Arab, deny Muslim and be a Christian. So it's not inviting Muslims. It's providing a place for Arab Christians, whose experiences and culture may differ from the more common Western culture that mostly dominates the site, to have a place to fellowship. Exact same as the Indian forums.
What you're saying is akin to saying being Indian makes you a Hindu and having an Indian room is inviting Hindus. Being Arab does not make you Muslim anymore than being Indian makes you Hindu or being Irish makes you Catholic. It may be the more common decision, but it is still a decision.
Further, Robo makes a point to not create areas that are intended to draw non-Christians to the site, though some have suggested ideas that may encourage that.