Yes it is perfectly legal...that is why they have security locks to prevent people from entering their networks....if you dont want to share...then lock it up...;)

Yes. I know a friend's friend and she steals her internet from her elderly neighbors because they don't put a security lock on it so they're just relying on their internet to get it for free.

No. However, the exact extent of the illegality has not been settled in many jurisdictions (more on this in a second). First off, your argument about your neighbor’s wifi connection invading your property has been tried and failed. You have no right to object to radio waves crossing your property. The only exception is if the waves being transmitted have not been approved by the FCC for the particular use. Second, there has been some debate dealing with whether or not sharing the wifi connection with a neighbor is stealing from the service provider (basically the same issue as slicing into your neighbor’s cable line, which is illegal by the way). Although this is not a settled issue, there is a strong argument that sharing wifi is different than sharing cable. Namely, your wifi provider gives you a specified bandwidth. If you go beyond that bandwidth then you are charged extra. Thus, when you purchase wifi you are actually purchasing a set amount of data transfer. Also, it has been held that you can legally intercept any radio wave; it is just illegal to take measures to decrypt any signals. However, it is illegal to transmit a signal to a device with the purpose of interfering with its operations. You will note that using your neighbors wifi requires you to both send and receive signals. So what makes it illegal? Besides the above controversial reasons which have yet to be tested in court, it is well settled that consuming the bandwidth of somebody else’s service is theft. Just because somebody does not lock their door does not mean you are invited in. Similarly, just because you don't lock your wifi connection does not mean that other people can use it.