Because what they are doing is any better. 7 COPS hurt.

http://www.nydailynews.com/ny_local/2009/01/12/2009-01-12_10_cuffed_7_cops_hurt_at_propalestinian_-1.html

Police and pro-Palestine protesters clashed Sunday after a rally in midtown, where demonstrators expressed outrage over continued fighting in Gaza.

The scuffle erupted as hundreds of protesters spilled into the streets on Eighth Ave. and police struggled to push them onto the sidewalk, witnesses said.

"The police started spraying people with Mace or pepper spray," said Steve Sherman of Manhattan, an antiwar activist who watched the melee. "People were falling down from it."
The fighting got worse when an Israeli supporter snatched a Palestinian flag from a man and threw it to the ground, a source said.

About 10 protesters were arrested and seven cops were injured, police said. Two NYPD lieutenants and five officers were taken to Bellevue Hospital.

The confrontation capped a day of rallies in New York and around the world over Israel's attacks on Gaza. "Bombing won't solve anything," said Yasser Toom, 22, of Sunset Park, Brooklyn.

At a pro-Israel rally near the United Nations, politicians including Gov. Paterson defended the country's actions.

And My Thoughts:
While i do believe in our right to demonstration, what i don't believe is the irrational these people have that hurting OUR Cops is any better than Israel bombing Gaza (which i do believe in btw).
Seriously people! You protesting in NYC will have absolutely NO affect what-so-ever on the bombings over there thousands of miles away! Are you kidding me? It's one thing to let our Leaders know that you are upset.. Write a letter, make a sign and go stand in D.C. somewhere, but please don't come to NYC and hurt our Police Officers because you and your ancient arch enemy can't get along. THAT is why you all live here! Now if you don't like that cohabitation i suggest you go back over THERE where you can do something about it! Like, oh, duck from the bombs.
For once, get out of your own selfish heads and let it go already. Yes, it sucks that innocents are dying, but that is your own faults (or rather the people you are protesting for), for not uprising against a terrorist organization. Had you all done that.. a more normal Gov't would be in place and not Hamas. So don't take it out on our COPS.