"stealing"? The WiFi is configured as "unsecured". Isn't that the same as stringing a bunch of 10baseT cables randomly out beyond your property line? Would someone attaching to those 10baseT cables be "stealing"?
No doubt "stealing" is not involved if the WiFi is "secured", even if that security is the simplest.
The difference is that one indicates "no tresspassing" (secured) and the other does not.
So is using an "unsecured" WiFi "stealing"?
if the person or business who pays for the wifi connection is openly allowing use / giving you permission then no.
but if they failed to secure the connection, did not give you permission and you know that you are using it then yes.
the post you refer to the OP seems to know that they are using a service they did not pay for and it's from a nearby home - then yes they are stealing.
I think a better way to look at it is like this, say a house has an open door, you walk in and attach an electric cord and run it over to your house and use the power. that is also theft. same thing.
the "right" thing to do is go ask if you can use it, also tell them they should put a lock on it to stop others from taking the service w/o asking.