TV usually works, for the most part, on a downstream channel though some pay-per-view, and off-site DVR features may use upstream to communicate with head-end controllers.
Cable systems were originally designed to have a few large transmitters broadcast to several thousand receivers. To have such a system work in reverse requires a much cleaner path, and simply does not have the same amount of bandwidth allocated for upstream communication.
Furthermore, while a bad splitter or run of coax might degrade signal slightly near a home's television sets, since the noise is not amplified or converted to light to run on fiber, and the inbound signal is strong to begin with, the degradation mightn't be noticed. However, in the reverse-path situation, all the ingress noise created at the customers' homes gets amplified and combined before getting back to the CMTS; enough noise to drown out one cable modem at one house might be enough noise to drown out all cable modems on the whole node.