[Symmetry]

Something people run into all the time when talking about toxicology is that generally people tend to believe that either a substance is safe or not safe, irrespective of the dose. The EPA says that 10 ppb of arsenic is perfectly fine in drinking water but I'm pretty sure most people would be horrified to learn that their water has any arsenic in it at all.

People naturally apply the same heuristics to nuclear fallout for the same reasons. Fallout seems equally bad whether it's a 10 S dose or a 10 uS dose. And just as arsenic poisoning is a real thing that happens, deadly fallout was and remains a very real danger from nuclear war. Any nuke will produce some but if you set a big one off near the ground, such as to hit an ICBM silo or a sub pen or a command bunker, and you'll produce a very large amount. At the height of the Cold War a US first strike on the Soviet block was expected to produce up to 100 million casualties in Western Europe from fallout if the wind was blowing the wrong way.

Given that magnitude of danger it's no wonder people hear "fallout" and think that they're all going to die.