How does wireless internet in large cities work?

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I was thinking about this because I can often log onto various accounts in the air that aren't locked, using my laptop. I started to think, wow, here I am in this small city with like 105,000 people and I can get onto various accounts. How does it work in large cities like NYC or Seattle, etc.? With so many accounts floating around in the air, how does it work that everyone doesn't just find the few people out there that don't put the firewalls and locks, etc. on their wireless and just use them? I know here there are a lot of cafes and such that also offer free wireless connection so what's to stop people from using those free systems in a big city? Just curious.