This seems to come up occasionally on the various airline forums. UA has a very similarly worded policy IIRC. There's a thread on the Travel Photography forum about this - and probably some more threads there too

http://www.flyertalk.com/forum/trave...s-flights.html

The argument over where it is and isn't "legal" to photograph seems to have heated up lately with security being the supposed criterion. I seem to recall some recent articles about people being harrassed for taking photos or videos of cops arresting someone out in public, which should be clearly legal in a public area. I don't know if an airliner qualifies as a public area but it should in my layman's opinion - it is a public conveyance.

Most of my contractor's plants prohibit photography inside for proprietary reasons (which is a company policy not a law, with some exceptions). I suppose the airlines might argue proprietary restrictions but I'd be at a loss as to what is secret about AA's 757 vice UA's 757, etc. Do they not want the competition sneaking a look at the dried up beef stroganof?

As mentioned upthread the security excuse is lame. The layout of any airliner is well known and easy to find even on the airline's website.