Role of FBI in domestic terrorism cases

Trevor Aaronson has a remarkable article in the September/October issue of Mother Jones magazine, highlighting the extent to which the FBI relies on sting operations to catch and convict alleged terrorists.
He says that in the absence of any externally organised terrorist plots in the USA, the FBI is preoccupied with targetting 'lone wolf' operatives. 
Aaronson says in many cases paid FBI undercover agents - of whom there are said to be around 15,000 - often propose a plot, provide explosives, lead a target in making a fake oath to al-Qaeda and then organise the arrest and subsequent press conference.
He says nearly half the terrorism prosecutions in the US since 2001 involve informants, some of them earning as much as $100,000 for their efforts. Such sting operations resulted in prosecutions against 158 defendants, of which 49 defendants participated in plots led by FBI agent provocateurs. A full list of the cases can be found here.
With only three exceptions, all the high-profile domestic terror plots in the last decade were actually FBI stings, including the Washington Metro bombing plot, the New York subway plot and the attempt to blow up the Sears Tower. As one former FBI officer concedes in the article, the uptick in successful terrorism convictions might not be evidence of a growing threat, so much as a greater focus by the FBI.