Well I am happy to report that I did some research and the rule has been removed from the 2016 edition of the code. The justification as given:
http://www.nfpa.org/Assets/files/AboutT ... Report.pdf (page 5)Current boarding bridges with glass walls/windows have been successfully tested in accordance with 6.4. Recent information developed for the committee suggests that use of glass will not significantly impact the probability of a successful aircraft evacuation. This data includes limitations of current bridge use, fire loss history, fuel drainage requirements, crew training in emergency evacuation procedures at the gate, and aircraft human factors studies.
Hopefully this means we will be seeing glass jet bridges in the USA in the future! I'm looking forward to it as, though the time spent in them is usually short, jet bridges can get claustrophobic and uncomfortable.