I agree with your post. That being said, it wouldn't surprise me if:
A. A pro-abortion person mailed these letters, or
B. These were just amongst the tons of hate mail they probably get all the time, but the PP people freaked and called the FBI to have them tested.
Of course, there is no proof of this, I am just saying that I wouldn't put it past them.
Yeah, there has been a lot of recent precedent of radical Leftists perpetrating such fraud. Unfortunately the archives are down right this moment or I would look up a couple of them for you.