He was walking on a beach and they ran a check on him to find that he owed $25k? I don't know much about California, but can you do that? Or is there something else missing from the story?
What is the problem? If they recognized the dude and ran the check and then approached him, where is the problem? Why look for a problem where there is none? Be happy they caught up with him.
The problem is it's not even like he was in his car. He runs a red light, they pull him over and find out he has a warrant, I get it. He gets kicked out of a bar for being drunk/belligerent, I get it. He's smoking pot in an alley, fine. But so far the story is "Matt Barnes was walking down the street and got arrested for something completely unrelated to walking down the street." Normally you don't get arrested for outstanding warrants on traffic misdemeanors unless you've done something else to call attention to yourself. Maybe they show up at your house, but that doesn't seem to be the case here.
California is a huge state with an overwhelmed legal/penal system, and assuredly tens of thousands of people walking around the streets with similar outstanding warrants for non-violent, victimless crimes. What caused police to even bother checking on Matt Barnes while he was walking down the street to get him caught? There's more to the story.