It wasn't like the street they were on was empty and, even though this was New York, it was only a matter of time before someone called the cops on an entire group of people brawling in the street. It just maybe took a little longer than it would have anywhere else.
In her own fight, Cassie was in a very bad position. At first she'd managed to knock down a couple of her attackers and was keeping the others at bay until, in a moment of distraction, she glanced over at the other battle and ended up pinned the ground while several feet launched kicks at her. The wail of sirens cut through her groans and the assault on her was halted while the women looked to their leader. Cassie shouted, "Go!" at Billy before she was shrinking down out of sight. The Young Avengers duo was way over their heads in this and it was totally Cassie's fault. Retreating would probably save their lives.