It is true that ATS is effective in a wide range of speeds.
Limiting trains to 79 mph clearly suests that ATS has been discontinued. There are some "magic" numbers that explain certain maximum speed limits.
Excepted track - 10 mph freight; no occupied passenger cars/trains; no more than 5 cars hazardous material.
Class 1 - 10 mph freight; 15 mph passenger
20 mph - maximum speed over grade crossings without ditchlights
20 mph - restricted speed including yard limits w/o signals (stop within one-half the range of vision not exceeding 20 mph)
Class 2 - 25 mph freight; 30 mph passenger
30 mph - Event recorder required on lead locomotive
Class 3 - 40 mph freight; 60 mph passenger
49 mph - maximum freight speed in dark territory
59 mph - maximum passener speed in dark territory
Class 4 - 60 mph freight; 80 mph passenger (freight locomotives typically have a maximum speed of 60-70 mph due to gear ratio)
79 mph - trains require ATS,ACS/CCS or ATC above this speed
Class 5 - 80 mph freight; 90 mph passenger (much of the Surf Line)
Class 6 - 110 mph
Class 7 - 125 mph (NE Corridor)
Class 8 - 160 mph
Class 9 - 200 mph