Earlier quoted context omitted.
But the FBI busts people for (Islamic, left-wing, etc.) terrorism when the plans aren't serious, or the FBI themselves will help the would-be terrorists come up with plans, money, supplies, etc..
I can't remember anybody busted for "left-wing terrorism" lately... As for Islamic, there's some reason to take them more seriously, given the recent history.
There are some Cleveland Occupy kids sitting in jail right now whom the FBI helped every step of the way.