Any group of people, no matter what color...
Praying loudly prior to boarding a plane- no matter what religion- would make me nervous... BUT it isn't against the law... And really its hard to see how it can be legit to take them off the flight...
I once got seriously creeped out on a flight where the woman next to was obviously very religious and kept talking to me, her topic of discussion kept drifting back to death and how it could 'happen at any moment' and how she was at peace with it... this was many years before 9-11....