If you are breaking the law and you get told to move on by the police, and don't comply, you risk being arrested. Should you resist arrest, it will be done forcibly. They were all adults and knew what they were doing. When you safely arrest somebody who is resisting, it takes multiple officers, and they get you face down, on the ground, sometimes with an officer on each limb. That is done for the safety of the person being arrested. It may make for a great front page photo, but they are doing it correctly and safely.
I was once stopped by armed police while on a motorbike in London, weapons raised. I did exactly as they said and was handcuffed. After 10 minutes they realised that they had the wrong person and apologised. That's how the system works. For everybody's safety they put you in cuffs.