You know what would have been cool?
If they had metal detectors and had police pat people down privately before entrance... you know, to reduce the risk of someone actually shooting!
She cancelled because, despite the fact there was a threat made against her, the school went "oh well nope we're not going to do anything about it."
If the threat was made, the school should have put in measures to reduce the risk. If at that point, she refused to speak, with the idea that "well as long as ANYONE makes ANY sort of threat," then you might have a point.
I agree that people say stupid shit on the internet all the time, and rarely anything happens. However, refusing to even try and mitigate the situation, and instead blaming her for being concerned (in a country that has been rocked by multiple mass shootings very, very recently)... is really condescending.