They were shocked because they saw how ruthless the government was towards the marchers. The marches were peaceful and included students of various ages. The police beat them up, released dogs on them, and used water hoses to send them back which was shocking considering that they didn't defend themselves and were peaceful.
The United States is a common law system, which means law is built on cases that came before it. So a Justice, and any Judge/lawyer, looks to precedent when deciding how to proceed in a case.
