Sometimes I let these people stick around a LONG time due to questions like this about why they're banned. Because by doing such folks eventually learn to not scrutinize such decisions.
But when I let them stick around, then we get reported posts about the Muslim who's in the forums promoting his Islam and such. Then we get accused of "promoting Islam", or "letting new weak unbelievers be exposed"...and on and on it goes.
Sorry I can't please everyone 100% of the time.