I think that ANY TV with a tuner will have the capacity for NTSC. There are millions of homes that are connected to cable systems that currently output NTSC, many don't even use converter boxes. Also some low powered stations still haven't converted yet. Most people who buy TVs have no idea what ATSC or NTSC is, they just want their TV to work.
Cablecos generally use QAM, a version, I suppose, of NTSC. Channel assignments and spacing are a bit different than OTA NTSC. My TVs let me hook a coax up, and I had to select "air" or "cable" - reflecting the different schemes.