
two main reasons, the main one, NTL:tw:VM didnt see fit to include the software in the STBs to allow you direct access to the transport streams.
this is were all the data is kept, be it video,audio,EPG,sub titles etc.
second, the fact most DVD recorders in this country dont have TCP:IP rj45 interfaces to connect onto the ethernet rj45 plug of any DVB* device that does have such software included.
its mostly a simple exercise to include a html web interface inside such devices (they are just simple SOC [SysyemOnaChip]based PCs doing a single job after all) to gain access to the transport streams and control/save/stream the content inside.
much the same as your generic BDA driven DVB-T and DVB-S cards today , but as i said thats VMs fault for not including it.