Not trying to start a fight here, but the problem with that line of thought is assuming that the human can only see down 600 feet as well. My eyes are pretty good, and during my younger speeding (statute of limitations have long passed) days, I would look about a mile down the I-5 for signs of cops. 1 mile - 5230 feet. Pretty significant difference in awareness vs. reaction time. Wouldn't try that again at my age, but just pointing out that TACC seems to be inferior for some stretches of highway (namely long straight ones).
My guess is practicality. Tesla probably doesn't want to encourage high cruising speeds, because it'll drop the range down like a rock! No numbers available to back that up though.
It's certainly valid that in good weather in daytime you can see further than the radar can. I would hope that you'd still be looking that far ahead while driving on cruise control, too, and could take action for the car if it became necessary...
You may be right that the consumption numbers over 90 mph are so ugly that Tesla didn't see the need for TACC to operate that high. I'm also not 100% certain that those are the specs of the Model S radar - it seems to be the most likely of the three ACC radar vendors I found from the pictures, but it could be something completely different.
Walter