No, they won't allow it. The main issue is that the cable network is susceptible to noise and the biggest contributer to noise on the network is dodgy connections/devices connected to the cables in peoples homes. The easiest way to control the network and keep it functioning correctly is to control the types of devices that can be connected to it. That is why you can only rent the set-top boxes and not just buy any old cable set-top.
Allowing this also leaves the network more open to piracy etc.