I'm not sure what type of answer you're looking for, legal one, a moral one, or a semantic one?
Its possible your neighbor doesn't mind if people piggyback on his Internet access, so it would be the equivalent of him putting out some broken couch on the sidewalk for anyone to pick up whom wants it.
But you don't know that, so its more of an equivalent to a fruit tree overhanging his property line, and that involves some sort of moral judgment. If it were a tree of a neighbor you don't know that lives a block away, if it were a neighbor next door that you know well, or a neighbor next door that you don't know very well, but the tree isn't overhanging the sidewalk, but rather into your property, and you pick it from your property. Same sort of moral judgment applies to Internet piggybacking. Overall the best ethic is to use your own wifi, but in some cases using others shouldn't be such a big issue.
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.
Yes, but their electricity costs them money by the watt, while most IPs won't charge with bandwidth, you're also opening their door and stepping inside which they might consider a violation of their property, which you aren't doing with wifi. But still... when I'm in a public place and my cell phone isn't charged I often look for an outlet on the side of a commercial building to plug it in. Nobody cares.
At any rate, there's also a wider definition of stealing which has no moral context to it. Consider expressions like "I stole a glimpse", "I stole the gift into the house," etc. which just means to take surreptitiously.