You cant honestly believe it is because they are black.
The police are hesitant to arrest most protesters, regardless of race.
Police were very restrained with the middle-class youth protesting fox hunting years ago, despite it getting way out of hand many times.
Protests are tricky, take the heavy handed approach and they become martyrs while making the cause more radical. Prevent the protest from starting and it looks like you are infringing on peoples rights. Stop the protest from escalating and it seems like governments are covering things up.
There is no winning way to deal with a protest like this, all you can do is arrest them when they cause an appropriate amount of disruption and hope the rest fizzle out