One could argue that the heuristics are for what a Muslim looks like. In that sense the tragedy is that that Sikhs don't really have anything to do with Islam at all. That most Americans think of a beard and turban as a fool-proof indicator of the person being a Muslim of course shows a lack of consideration.

How relevant moderates are to extremism is a separate question (and one that is largely orthogonal to the religion itself) but it's fair to say that no matter how little a moderate has to do with extremists, someone who isn't even a member of their religion in the first place is about as undeserving of hostility as it gets.

I would argue the greater tragedies are that ethnicity frequently gets conflated with religion (which really serves no-one, unless your religion is incestuous) and that most people are too ignorant of other cultures than their own to tell them apart.

Heck, there are still Americans who have a hard time accepting that Obama isn't Muslim just because he's not white (as if non-white Christianity hadn't ever been a thing in the US).