We weren't alive then.
I agree a 30% cutoff is a bit harsh, but increasing oxygen percentage, AND pressure can easily cause central neural toxicity, as well as increased risk of fire.
At one atmosphere 40% starts becoming toxic, and 55% becomes intolerable, at half an atmosphere you can get to 70% without starting toxicity, with 95% still being somewhat tolerable.
But with increased atmospheric pressures 30% and under starts being toxic. I'm guessing steve picked a number that was a bit of a compromise. Maybe more complex toxcity mechanics is justified considering how the rest of the gasses are treated now.
I'll attach the chart I found just for reference.