When I first moved into this apartment complex years ago, there were 4 wireless networks (besides ours), none of them locked. Now there are 6 networks, 5 of them locked.

Anyone with a wireless modem has checked for other networks, and although it's infinitely small, it does put a technical strain on the network.
Now that we know just about everyone's done it, we can discuss scope. :)

If you use someone's unlocked network to the extent that you hinder their internet experience, you are partially wrong, but so is that person. Hundreds of internet profiles were hacked recently using the password "god", "123456", "Michael", or various other easy names/word combinations. To keep yourself safe, you must learn to protect yourself from simple attempts. It's the real-life equivalent of buying the brooklyn bridge.

Now if you're actively hacking someone's internet that most certainly has a decent protection, now we're getting into where the law should start coming in. This is the same as burglary. You do not belong in that area, you are taking something that doesn't belong to you and making someone else suffer. This is wrong, and there will be stronger and stronger laws to fight this in time.