Continued use of this sort of tactic, though, not only undermines the AP, it also undermines the tactic.

That is, if criminals begin to distrust reporters as being potential cops, the FBI loses that mechanism to gain trust, -and- the AP loses the ability to talk to those society deems unlawful. Unlike those other methods you mention, which don't effect society at large, just the individual (and can be weighed solely on those merits).

So this is a procedure that is doomed to make itself obsolete as a criminal trapping tool -while also having repercussions on a facet of democracy-.

Imagine if this was a common occurrence, happening prior to the Snowden leaks. Would he have leaked? Or what about people who leak drafts of the TPP? Giving a voice to those the government deems criminal is a necessary function of a democracy, and this undermines that.