This concept applies to any country where small amounts of ammonium nitrate are easily obtainable as fertiliser in small quantities, and it would appear this is okay in at least some US states.
I was watching a UK thriller TV show where sniffer dogs making sure an event would be safe came across what appeared to be some accidentally dropped ammonium nitrate, indicating that a substantial explosive device might be in the building.
So it occurred to me that to disrupt a high-profile event, legally, one simply had to accidentally drop a small enough sample that dogs would pick up on. A failure to find any actual security risk could still possibly/probably mean the event gets cancelled out of an abundance of caution.