Because I haven't really seen people claim forgery when the letter is actually real. That is a problem that can easily be fixed 
If we have people starting to claim that real letters are forgeries, then claiming forgeries are real letters becomes easier too.
Good point
However, claiming real letters to be forged might prove just ask risky for one's own reputation as claiming forged letters to be real.
Then again, it should be pretty easy to gain (or buy) the support of someone supporting your claim?
