"Since the majority of Indians are Hindus and not Muslims - why would you describe someone leaving a mosque as Asian instead of Muslim?"
Because the description 'asian' is used as an ethnic description. It is not in any way a synonym for 'muslim'.
Your paragraph on why American police tend to give the ethnic description 'black' applies equally to why British police tend to use the ethnic description 'asian'.
Maybe you're not getting my point. When "Asian" youths come boiling out of a mosque - it gives the impression that they might be a bunch of Chinese kids. I don't understand the Brit's aversion to calling Muslims Muslim. It is not the "Asians" who are working toward a world wide Caliphate - it is Muslims who are. It is not "Asians" who want world wide Sharia law - it is Muslims.
Your insistance on calling Muslims "Asians" gives the appearance of Political Correctness taken to an extreme - almost like you are afraid of offending a Muslim. If I was a Korean or Japanese living in Britain - I would be highly offended when I heard rioting muslim youths described as "Asian".
The word Asian is to broad of a catagory. The word "Muslim" is more specific. And it is the "Muslims" causing trouble - not "Asians".
Some of the Muslims causing problems are not even Asian - there are Brits who have converted to Islam. If a bunch of English born converted Muslims start throwing rocks at cars - you can't very well call them "Asian", can you? You would be forced to call them what they are - "Muslims".