I'm not very knowledgable about the equipment used by cable companies to connect homes to their central offices. However, I imagine you are sharing some sort of switch with at the very least, people on your street. I further imagine the cable company keeps logs of MAC addresses that transit traffic through those switches. It might even work on a lower level than that - i.e. the cable company can log all MAC addresses coming in and out of your house, which should normally just be the cable modem given to you.
It's certainly detectable if the person cloning your MAC lives in a different neighboorhood - clearly then the cloner is NOT using the same physical connection to the network.
All cable modems have firmware and I would even further venture to imagine again that the MAC address assigned to the cable modem is assigned in firmware, and if you can get access to the firmware (which is likely possible by putting the cable modem in some type of test or debug mode - possibly through JTAG headers on the cable modem's motherboard or directly connecting to any NAND flash chips on it) - you could probably figure it out - if you are familiar with the CPU architecture of the cable modem and defeated any encryption.
So it is entirely possible that you took a cable modem you found or stole one from a person's house, got into the firmware, found out where the cable modem MAC address is, and then hacked your own cable modem's firmware to assign that MAC to it. The ISP can't really stop you from doing this but it can detect it, unless your cable connection is on the same physical switch as the person you are cloning. Which is unlikely unless you live on the same street.
I could be wrong about all this.