Ethics: Is it okay to use an unsecured residential/business/mobile hotpsot Wi-Fi?

I was having this discussion with my girlfriend about whether it was okay to use others' Wi-Fi networks if they were unsecured. I noticed that in university (because the university Wi-Fi is terrible) people are using their phones as hotspots, and are often leaving them unsecured. (Note: I don't use other people's hotspots/networks because I am wary of my data being tracked by the owner)

Now, my point was that most routers and personal hotspot functions on mobiles have a pre-determined password. Therefore, removing this password means that your router/hotspot is open to everyone to use.

My girlfriend, on the other hand, suggested that not everyone is as technically-proficient as I am; they may not have understood what removing a password does. She also stated that it is stealing - I am gaining unauthorised access to someone's connection and am using up their usage and effectively stealing their money.

So I want to know what you guys think about the ethics of using an unsecured Wi-Fi connection. Is it good? Is it bad?