I see WiFi as an extension of your land and the same laws that apply to trespassing should apply to your WiFi. It is not generally considered trespassing if the owner’s usage is unimpeded, the owner did not make a reasonable attempt to block you (log-on note, encryption, etc analogous to fence, gate, etc.), and the owner did not tell you to stay off (negative consent either verbally or in writing). If you break encryption/password then it should be illegal, you bypassed “reasonable measures” securing it. If your usage is so great that it impedes the owner’s usage then the owner must still investigate to point you out. If you spy on the owner or mess with his settings/etc. then you’re either committing wire fraud or trespassing.

Someone can not legally walk into your home even with an open door or bypass your fence, gate, or no trespassing sign. Those are considered reasonable measures to inform (the home is a separate law). However, without a reasonable measure of negative consent, your lawn is open game for squatting, just like your WiFi.

[Some use a "what if I leave my car unlocked and someone takes it" argument.] The "steal your unlocked car" example does not hold due to different laws. You own your car by agreement of the state government via a Title. If there is no record of you possessing that title then you do not own your car, in which case the person who last possessed a title owns it, unless sold to an undisclosed party, then whoever possesses it or a bill of sale owns it.

WiFi that is unsecured by ignorance (didn't know it had to be secured) is still open WiFi. Ignorance is not a valid legal defense in the USA. Some courts may allow it, but it is on a case-by-case basis, but generally it is assumed the individual is responsible for knowing all legal ramifications of his/her actions. That is part of being an American citizen. You can not prove ignorance. I could run someone over with a car and say I didn't know it would kill them. Unless someone could prove I had a mental disorder preventing me from understanding that very basic idea then I'd go to jail for at least manslaughter. This idea usually crops up in traffic cases where someone didn't know about a local law.