Everything I've googled up on the 612 mentions only ATSC. I see no claims of being able to support QAM as most cable systems use.
Of course, some cable companies still broadcast some basic channels in NTSC because there are old people with TVs from the 80s that still work. But what I have googled up on the 612 doesn't mention NTSC either.
As hddn says, the good reason is completely different tuner. COMPLETELY different tuner. The OTA adapter is named very aptly. It's called that because it tunes to the channels Over The Air. It does not tune cable, it does not tune satellite, it does not tune radio, it does not tune a piano, and it does not tuna fish.
QAM is a TV feature as part of the tuner function. It would pick up local basic channels without a cable box, similar to old tube TV's that were 'cable ready'. Cable companies don't like that so now even local channels are scrambled and you need a box for every channel. So, even if you plugged the cable system co-ax directly to TV ant in or HDMI1, you'd get 'nothing'. Connecting a cable system to a DISH receiver would be the same as connecting your garden hose to your kitchen can opener.