So theoretically, I could listen in on my neighbor’s wireless phone calls if I could figure out how to do it. In the old days, scanners could pick them up.
Heck, theoretically you could chime in on the conversations. (I refuse to admit I’m old enough to remember party lines. That was way before I was born. Decades. Eons. Well, OK, but we only had the one (sob!))
IIRC we had one in the 80’s that you could listen to using a short wave radio.
If you listen to their calls or monitor their internet traffic, you’re stealing information from them - but it could be deemed similar to eaves dropping.
As long as you don’t slow the owner down, you aren’t harming him or the ISP by using the wi-fi signal.