Yes I know there are Christian Arabs.
Yes I know there are Christians who associate happily with Arabs of all faiths.
But only a fool would expect the percentage of Christians at an Arab festival to be anything less than far below the norm for other gatherings in the US, so the microcosmic answer is "not there to interfere or remonstrate with the loony Christians".
The more germane macrocosmic answer unfortunately is not that different. There are plenty of Christian groups doing very good things of course - lots of soup kitchens with saints names for a reason. There are even some Xian groups trying to present at least a somewhat gentler and more inclusive sociopolitical view of and from Christianity. But they do so in far fewer numbers, far less aggressively and far less consistently than the loony kind.
Why? Partly it's pure numbers. Apologists talk about Mainline Protestantism a lot, but despite the name that's a theological distinction not a statement of majority support. They are far outnumbered by more reactionary denominations. And even Mainline churches run the gamut that ends well into loony territory.
Partly it's enthusiasm for this kind of activity. Bishop Robinson says some nice things. He has far more supporters than Pat Robertson had when he started buying media stations that built him a massive megaphone and explosive growth in that support and quickly made him a leading face of US Christianity. Bishop Robinson is not buying media stations. No kinder gentler loving tolerant type of Xian seems to be. They are not giving more to charity or volunteering more than conservative believers as is often suggested, but they certainly are giving less to fund modern outreach and media access. Who then speaks for the faith by dint of megawatts and audience reach?
Partly it's because hate sells. It's simpler. It's an easier pitch to tell an uninvolved blank slate audience that gays are filthy sinners and Muslims are heathen terrorists and atheists will fry in hell than to explain the sparse biblical injunctions against homosexuality do not come from Jesus, or that Islam is just an alternate revelation of the same god, or that hell is a metaphor stolen from Zoroastrianism. "Hate the other!" penetrates the mind of the mouthbreathing masses far more deeply than "the other is far more like you than he first appears, and is merely the result of slightly different biological and/or social factors".