When "Dissent" Becomes "Threat"

ABC News:: "School Talent Show Draws Secret Service: Colorado Band Singing Dylan Song Seen as Threatening President Bush."

"Threatening the president is a federal crime, so the Secret Service was called to the school to investigate."

Please, in this day and age, define "threat"? When singing an anti-war song merits a Secret Service investigation, tell me how "threat" is any different than "dissent" for these people?