To me a thief is someone who knowingly takes something of value that doesn't belong to them. Someone who uses a cable signal that is piped onto their property (without their permsision I might add) may be stealing the signal... but they aren't necessairily a thief.
No offense, but this sounds like a justification to sooth your own conscience. It is common knowledge that the same wire carries both the cable TV signal and the broadband. By purchasing the broadband service and requesting that the that line be brought in to your home, you are honor bound to not pirate the other signals on the wire. They are not delivering something to you that you didn't request, you requested thier wire, so you are obligated to pay for anything you use that's on that wire.