I wonder if you would be liable for the service fees if/when they discover your free cable? I wonder if this could be considered "cable theft"?

Technically, yes. Some cable companies have done exactly that, and have recouped all the fees for lost service, sometimes into the thousands of dollars. (I worked in the cable industry - on the programming side - but we had a hardware division for a while until it was divested and I can assure you that they did it.)

I always wondered if a defense of "I didn't ask for it" wouldn't work. After all, if a company mails you a product which you did not order and then sends a bill, you are under no obligation to pay for it. Even more, you can use the product; you are under no obligation to send it back. I would assume that someone has tried this defense, but I know of cases where there was summary judgment and payment for the cable services was due immediately.