Laws aren't necessarily logical or fair and you might never grasp the danger of that until you are on the wrong side of one. Unless you are purposely trying to get arrested to make a protest, stop chatting with cops! Their policy is typically "arrest anyway, get them in the system, let the courts sort it out later".
Cops know they can get more out of you by being non-confrontational and mirandizing you well after you've told them everything they need to know to make up a very damning arrest report - you'll never get a warning.
What's really surprising in this case is only that this particular person didn't grasp the concept. He probably got to walk away only because they knew the ipad data could make much better evidence to arrest others than forced testimony. But they obviously used his arrest to leverage him giving it up without a fight.