Not that crazy. 20% of 300mi is ~60mi which is slightly more than when the low fuel gauge lights up on most gas cars. And most people refuel gas cars long before that light comes up, when there's ~100mi of range left. Why is such behavior suddenly so crazy when you translate that to EVs? I'll grant you that it's unnecessary in most situations, but why is being a little more cautious than others about preventing a very inconvenient situation warrant the word "astonishing"?

Only last winter I was stuck in a snow storm due to traffic and road conditions in my EV. That route took an extra 10% off my battery than calculated due to running the heater. 10% is the bare minimum margin of safety I will ever accept, and I will not feel comfortable at all having only 10% buffer.

Also consider a gas car can be refueled with a can in desperate times, allowing you to get to the next gas station. EVs have no such equivalent. Best case, you're waiting for a tow. Running out of fuel in a gas car is inconvenient. Running out of battery with an EV is devastating. Considering the difference in consequences, it is well worth the extra caution.