Kas, it is not impossible to cancel NRA meetings. It's just unprofitable.
Ten day notice required for, Columbine happened eleven days before.
Yes, it wouldn't be impossible (even though you would have thousands of people driving through Columbine wondering where the heck everything is), and I do agree it could be seen as a bit insensitive to hold the meeting, but they did cancel everything but the required meetings. It's not like they came and partied, shooting guns everywhere, and detonating explosives.
The mail doesn't deliver within hours. It would take at least three or four days to 1) Print up cancelation notices, 2) Address them to the 22,000 people who were attending, and then 3) Mail them.
And then you have all those people who either don't read their mail, are already on their way over (roadtrip and all), or are simply stupid.
It could be offically cancelled within a day. The letters would be out in a week, and taking down the billboards and posting notices in the town would have informed the stupid people.
Face it; Heston didn't cancel the meeting because he really couldn't give a shit about bullet riddled corpses when he has live people to sell guns too.
Ahnk is right. If you're going to let technicallities be your excuse for promoting weapons for killing to those in mourning over death, you're really not trying.