How do you know it was going 80? Not 50 or 35?
or even faster, how about 90 or 120?
300 feet is nothing for a helicopter. But to counter point a slow moving helicopter, is the simple fact that a helicopter is MOST vulnerable when moving slow. According to the report, they were involved in drug interdiction operations. Drug cartels and their soldiers have no problems shooting at helicopters. Pilots know this so it is far more likely that the helicopter was moving with some speed rather than just sitting there waiting to be shot at.