Well, maybe it's because you're not actually looking and you're only repeating a talking point that sounded good to you at the time.
Here's the source (and here's the source for the picture at top). Of course, it's the New York Times, that liberal rag, so maybe they picked the one single spot where people had their American flags out. Or maybe they handed out the Stars & Stripes just so they could prove the yahoos wrong. Could be it. They're that evil.
While we're at it, Exhibit B:
Here's the source for this one. These were easy enough to find. All I did was a Google image search of "immigrant rally." I didn't even have to add "American flag" to get these results. KushiFact labels this one blatant lie.