Isn't necessarily about cheapskatedness as some have suggested - some passengers don't want to eat the junk the average airline thrusts at them, but none the less feel the need to eat something during a long haul flight.
As has been stated previously by several including myself, it comes down to terms and conditions. If its in the airline's conditions of carriage that you can't consume your own food, too bad - passenger has not got a leg to stand on, as it were, as these are accepted during the process of making a booking. However, the original poster said they got an e-mail from the airline, my inferenence from that being that this "instruction" was passed on post-booking. If so, and if the conditions of carriage don't mention passengers can't consume their own food, then passenger is within their rights to tell the airline what to do with its directive as consuming food brought onto an aeroplane is not against aviation law.
I can see the pre-departure announcement now - much along the lines of the "no smoking" directives already in force and aimed at those trying to evade the GlobeSpan Own-Food-Police: "Passengers may not consume their own food onboard this aeroplane. Consuming your own food is also prohibited in the toilet and washroom areas. The lavatories on this aeroplane have been fitted with food detectors which will sound an alarm if passengers attempt to consume their own food in these areas".
A