Many store owners don't like a lot of kids hanging around outside because it often scares away customers. They're also worried about vandalism and mass shoplifting where large numbers of people come in and overwhelm security. I saw that happen firsthand when I worked in a store during college.

I'm not condoning arresting hundreds of kids, I'm saying there may be more to this story than meets the eye. The store owner/manager might possibly have requested the police to break up the crowd and things got out of hand. As you said, bad things often happen when large groups of teens start hanging out.