Person of interest sought after car drives through security fences at South Carolina nuclear plant
be_excellent_to_each_other@kbin.social to 
abcnews.go.com
Police in South Carolina are investigating an incident involving a vehicle that drove through security fences at a nuclear power station on Thursday.
Well that’s horrifying, I always assumed someone attempting something this potentially destructive would have been put down 100 yards outside the gate
The article says he'd previously been turned away at the gate, so they knew what level of threat they were dealing with.
Fair, however:
So this sounds like he was inside the perimeter but potentially not within the boundaries of a higher security interior zone, to me. It may not mean that at all, but if it does it might explain (see my comment above) that they weren't authorized to use force under the circumstances.
Again, I recognize this is just speculation on my part, just explaining my thought process.
The restricted area is basically the entire grounds, excepting one parking lot and the front of what looks like an office building.
https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/full-text.html#part020-1003
If you look at it on Google Street view, you can see a "restricted area" sign on the fence right next to the highway.
I would guess complacency initially (can't recall a similar incident in recent memory), and also some of the language in the article lead me to believe that a particular threshold wasn't crossed. Maybe that threshold is the only level of threat under which they are allowed to respond with deadly force. (that's just rampant speculation on my part)
I get the impression they were private security not police, so that would explain them being less trigger happy.