A lot of misinformed people in this thread. I was an in-house cable technician (not a contractor) for 3 years.
OP, there is no such thing as having a house "wired for free cable." There is nothing wrong/illegal about wiring your house for cable. This is done all the time when new houses are built. Cable gets wired to any and all rooms in the house that an owner could possibly need someday for their TV/modem locations. All of these feeds meet up at some junction in the house, which is almost always in the basement near where the power/phone interfaces are.
An outside line enters the house at this junction in the basement. This is the feed from the cable system main line that actually gives you the cable signal in your home. The "interface" for turning this line on/off (which consists of simply screwing the end of the line into a port, or disconnecting it) is either above the street or above your backyard as part of the "telephone pole lines", or in a (locked) box in your yard somewhere (if you live in an area with an underground cable system).
When someone cancels their service for whatever reason (moving, don't want it anymore, etc.), the cable company is SUPPOSED to come out and disconnect your feed at the interface for your house. Sometimes this just doesn't happen. Either because of an oversight, or the technician responsible for the job didn't feel like climbing into that pine tree to reach the tap (interface) above your backyard, and just marked the job as done. So if a new owner moves in one day, and just randomly tried to hook their TV up to that port on their living room wall, it will still work for them as it did for the previous owner because it wasn't actually disconnected.
So in the OP's situation, both parties are essentially at fault. The OP is knowingly using the active cable for free, and the cable company didn't properly disconnect cable service from that house in the past. Since the OP is just using basic, analog cable, there is no hardware system or means of tracking this by the cable company... unless the entire cable system had a complete makeover in the 3 years since I last did that job. It's very basic stuff.
To further explain what happens in a situation like this (a customer that wants cable modem, but no TV service) -- if you order cable internet, a technician will come out and install the modem. Then they have to go turn your service "on" at the interface outside somewhere that I mentioned earlier. It is here that they will become aware that the cable feed to the house is already "on." Now, there's a good chance they just might not care at all or assume that you are paying for cable TV already. But a more thorough technician will look into the situation and just find out that the service wasn't disconnected properly in the past. They might ask you if you're using their cable TV, or they might just turn on one of your TVs in the house and claim they're just "checking something." Just disconnect your TVs from the cable line before the tech comes out and they can't prove anything about you using the service without paying.
BUT, what will likely happen is that the technician will do what they're supposed to do which is apply a filter to the line to your house that allows the cable modem signal to pass, but blocks all of the television signals. So if you order cable internet, you're 99% safe from being charged/prosecuted/some other crap... but you will in all likelihood lose your free TV access.