They can, but why would they when they haven't done that up until now? The problem is, they likely don't even realize that people want it since the "TV as a monitor" niche is so small. There's simply no logical reason they'd enable it. The only people who've ever made TVs that accept a 120Hz signal at any resolution are Sony (and maybe two from Vizio).TV firmware is easily updatable.
If the panel itself is 120hz, i don't see why they can't implement a workaround to make it accept the signal (via the box)