There are malcontents out there on the left who think flag-burning is a good thing. They aren't large in number, BUT . . .
From their perspective, Occupy is a natural home for them. They oppose the 1 percent and the general makeup of society. While Occupy might not want such elements, they definitely want Occupy. Here's a real movement with national traction that stands for some of the same things that they do. From their perspective, it's a perfect fit. They get to go out in public, do their thing, and get coverage and have people backing them up.
Just because the rest of Occupy isn't feeling them doesn't matter. It's an open movement. If you accept participation from anyone in the 99 percent, you have to accept them too. They are the 99 percent as much as anybody else is.
I really don't think it's a setup. I think this is real douchebags doing their thing. It's not called 99 percent for nothing. 99 percent includes about 98 percent of the riff raff out there