Stealing is stealing, no matter how you do it, and regardless of the person or person(s) you are stealing from.
If a bank leaves their vault standing open (whether by accident or by stupidity) it's still stealing when you walk out with a bag of cash...
Just because your neighbor's front door on their house is unlocked, that doesn't mean you aren't trespassing when you walk inside without permission. Whether or not you believe so, trespassing on their private wireless network (using their network without permission is trespassing) and using the Internet access they are paying for is still stealing.