Apparently the tape that had been applied to JonBenet's face appeared to have been used elsewhere before...so there's some issue as to how much adhesive was still left on it. If it had been on something else and taken off of that to be on JonBenet, then no, I don't have any problem with prints from JR and FW not showing up. You can only use a piece of tape so many times before it's lost whatever sticking power it originally had, and being applied to the face of a dead child over the bloody mucus running from her nose/mouth would definitely degrade any remaining adhesive pretty quick...as well as the adhesive being worn off when the tape was pulled off of whatever else it was on, like the doll - and the bodies of the dolls come in more than one shade of tan/brown.
As far as I know, the American Doll Theory is the product of a poster on other boards known as Pinker or Jahazafat, who had firsthand knowledge. More can be learned about this theory here:
http://www.geocities.com/pinker44/
The cord from the dolls would have been too short, but there's a shocking similarity in how the cord comes off the back of the dolls and how the cord was on JonBenet.