Running wireshark only shows packets that are delivered to your network interfaces. If people didn't want you to have that data, why did they route it directly into your computer's network port?
Using tcpdump after setting your wireless card into promiscuous mode will store all packets going over the air nearby. So, wireshark can easily be used to view tye contents of traffic that was not routed to your machine.
People set up radios and broadcast data completely indiscriminately? I would argue that is like yelling a conversation and then being shocked that people might overhear you. (Also it only gets certain packets depending on what network you're joined to, what channel you're listening on, etc.)
I agree about the yelling part. But, to clarify, you don't have to join a network, and you can always scan channels. Although chances are that most people around (e.g., at a coffe shop or airport) are broadcasting on a specific channel.
