You can see NTLs point, if a technician goes inside someones PC, and it goes wrong, even if its not his fault, its Claims Direct with most people these days.
So having USB means you can stay out of the "box" and out of trouble.
Supplying an ethernet card could have similar consequences if the home fitter fits it, and something goes wrong, ie electrocution or something.
They just turn round and say "NTL said i had to have it". Maybe this isnt the case, but i bet its been considered.