I don't see anything wrong with placing your passport in a seat back pocket while you are settling into your seat and placing all your other stuff in the overhead or wherever. The issue instead is not remembering to retrieve it immediately before sitting down and not placing it in a secure pocket within your clothes.
There are a couple things that bother me about the story. First, how did he return to London without the passport? Second, as pointed out by others, how did he get photos taken, copies of the paperwork to fill out, and all the other stuff to show up for an appointment to get his replacement? Also, are photos actually needed for a modern US passport replacement anyway? The passport you get has your photo printed on it, not glued to the page, meaning it was electronically stored anyway. Couldn't that photo be used when they print you a new passport?