There are no known criminals currently in Default Trust, any discrepencies always ends up with said user being booted off Default Trust. Do you have something against a user(s) in that list OP?
Depends,what your definition of a criminal is...As for now,you can only rely on the phrase 'Innocent Until Proven Guilty'.
Should debunked criminals such as scammers/extortionists etc. be allowed to have a place in Bitcointalk's Default Trust, thereby allowing them to arbitrarily add red negative trust to users they dislike or disagree with?
'Dislike or Disagree' - NO but if they have valid points for leaving the feedback which will judged as a right decision buy the community then it doesn't really matter if they're criminals or not.Their Feedback would be considered appropriate.
Should criminals be removed from Default Trust?
Criminals belong behind the bars,removing them from the DT wouldn't make a big difference.