They can't arrest you for "acting suspiciously".
They had no good reason to detain him. There are rules set forth, and he was following them.
If everyone thinks it's ok that he was arrested, how about you pass a law? You can't just start detaining people because you don't like something. You have to go through the process to pass a law.
Welcome to America, looking at some of these posts, it seems like most people would like our governmet to be able to do whatever they want with no process.
That's pretty communest if you ask me.