It's perfectly reasonable for law enforcement to allow some informants to commit certain crimes while attempting to shut down a larger organization. Simply reporting the number of times that this happens says nothing one way or the other about whether the FBI is doing a good job at making use of this power.
Personally, I'm much more worried about the times that the FBI and other law enforcement agencies engage in sting operations where they use such informants to urge people to commit legal activity and then arrest them for it. Some fraction of these informants may well be doing just this sort of thing, but the report of merely the number of informants doesn't say anything about that. Here is one example of such entrapment. Quoted from the above page:
The judge criticized not only the defendants, but also what she viewed as the government's overzealous handling of the investigation. Referring to Cromitie, she said, "The essence of what occurred here is that a government, understandably zealous to protect its citizens from terrorism, came upon a man both bigoted and suggestible, one who was incapable of committing an act of terrorism on his own. It created acts of terrorism out of his fantasies of bravado and bigotry, and then made those fantasies come true." She added, "The government did not have to infiltrate and foil some nefarious plot – there was no nefarious plot to foil." She said the defendants were "not political or religious martyrs," but "thugs for hire, pure and simple."