While it's true that there's no such thing as a specific amp for ATSC, there are amps that are better suited for ATSC than for NTSC.
With NTSC, all you usually care about is signal level, so it's just a matter of picking an amp with the best gain for your situation. But ATSC tuners are more susceptible to noise issues then to signal level issues. The amp only needs to have just enough gain to get the signal level on the good side of "the cliff". But you want the amp to be as clean as possible because if the amp is too noisy then no matter how much gain the amp has, it won't help. OTOH, NTSC will give you a viewable signal even in the presence of tons of noise, You might see some sparklies or other noise artifacts, but you'll still have a viewable image, unlike ATSC where you won't get anything or have so many dropouts that it's unviewable.
So you should almost always trade off gain for low noise for ATSC. And for NTSC, you should usually pick a gain that get's you as close as possible to overdriving the receiver and not really care about the noise.