Why are we allowed to make 9/11 jokes but not Columbine jokes?

Columbine is even OLDER than 9/11 but the terrorist attack that killed 3 thousand people is treated as lighter than the one that merely killed fifteen.

I guess because shootings are still happening now (actually there haven't been any in a while despite it being an election year, anyone notice?) but so are terrorist attacks and muslim extremist groups still exist so why can't I wear a NATURAL SELECTION shirt in public without looking like an butthole?

!schoolshooters i miss you