Sir the perpetrator of the Gurdwara attack (Wade Michael Page) themselves claimed they targeted this place because they believed this was an Islamic building and the people there were Muslims. The fool probably didn't even know Sikhism was a religion. And this is the same for the vast, vast majority of Americans and this ignorance is responsible for the disproportionate attacks on Sikhs in America because the image of Bin Laden with a turban is now the template of what a Muslim (and an extreme one at that) looks like. Ask the majority of Americans what religion a Sikh with a Turban is and they will likely reply "Muslim". I have a lot of family in American and I know this to be true, heck it's even partially true in Canada and to some extent in the UK (but not so much).
Indeed there are very blatant differences between a Gurdwara and Mosque and many distinguishing features of a Gurdwara that points out it is clearly a Sikh building, do we really expect a white supremacist scumbag who is willing to walk into a building and kill innocents of any faith to be aware of such things? He was already ignorant to the point of stupidity so why is it a surprise he couldn't differentiate between Sikhs and Muslims?
If you really want to see which minorities have it bad in America then, as
@sandy_3126 points out, look to the black minority in the US. Sikhs may have been on the receiving end of more than their fair share of pain but it is nothing compared to the relentless subjugation, discrimination, humiliation and persecution of the black community in America. Whereas Sikhs, like the wider Indian-American community in America, are one of the wealthiest and highest achieving communities in America, the black community (that have been in America for hundreds of years) come at the bottom of almost every list one can think of measuring success. This is before we even start talking about how they are treated by law enforcement in the US- I can't think of a similar case involving Sikhs in the US to the Mike Brown or Eric Gardner cases where the American police have killed unarmed black men with complete impunity.