I think there may have been an international gentleman's agreement about not putting external cameras on civil transport aircraft.
It may have been more of a Cold War thing, but I seem to remember running into that in a book at some point that there was a tacit agreement to not do that in the interest of not turning civil transport aircraft into mass produced intelligence gathering platforms, therefore in theory rendering them completely neutral in reference to needing to be taken into account in international sales/intelligence gathering.
I may be completely wrong or misinformed on that though; or the info may have been valid, but hasn't aged well taking into account how capabilities have evolved.