When you fire up your computer's wi-fi and it finds an open connection, there's no way for you to know why it's open. It may be generosity, marketing savvy. or stupidity. There are enough deliberately free hot-spots—Toronto Hydro once made one that covered all of downtown—that it cannot be said any time you connect you should know you're stealing. That is what passwords do for both sides of the issue. Besides, if the 'provider' has no cap, or has room beneath it, it would be difficult to assert there's a cost, or harm from connecting and certainly her ability to use her service is unimpaired. Hard to call it theft when you've still got the goods.
The wallet on the parkbench isn't quite the same. And it's rather different from finding a small stack of toonies there. Absolutely it is stealing to take either; they're not ours so it's theft. But the toonies are anonymous, and the loss to whomever has happened already. My bet is that for everyone of us there's a size of that stack small enough that they'd go into our pocket. We have treasure trove laws and finders keepers to guide us, as we have the computer laws quoted earlier, but they are a long way from truly answering the situation.
It's actually quite insightful to see the value of having the information superhighway as accessible as the physical sidewalks, streets and roads. They benefit all of us individually and collectively, which is why we pay for them—mostly—by taxing all of us, irrespective of our use of them. And no one suggests the tourists are stealing the streets they drive on. Our old wired telephone service was somewhat similar in that all residential customers were charged/taxed the same basic service rate, allowing pretty much everyone to have a phone and connect with their community without considering costs of every call. It made Canada the world's leader in phone usage, and Bell's supplier Northern Electric a leader in technology, Of course that system broke apart with the 'need' to maximize profits at every point. Service is so much better.
And now that mentality's driving the public sector as well, so what was public is now increasingly for sale. Like roads. Nonetheless, eventually we do come to see some things as public goods—police, most roads, schools, water, sewers, hospitals—and for them the ordinary definition of 'stealing' has to be tweaked. As we evolve to that view, some of us resist and lag, some bulldoze thoughtlessly forward, and some can't find the path at all. Which is why all of us, like the OP's brother, should take active care to protect ourselves, and not leave it to others. Can't assume we're all going the same way.
And there are those who have never thought how wi-fi got into the air at all. "What? Is that what wi-fi is? It just works whenever I turn on my 'puter, you mean it isn't free? How would I know?"