Why can't I take food from the plane when entering the US?

The airline is required by US law to follow a pretty complex set of rules relating to international garbage in order to prevent pests from entering the US. Many airports have incinerators on-site to comply with USDA requirements, and it's the responsibility of the crew to collect the trash so it can be destroyed.

They're not going to account for every edible item on the plane, especially prepackaged candies and snacks, and if you stuff the thing in your bag, they aren't going to start searching the plane for it. But if they see you with something that needs to be disposed of, they will want to ensure the regulations are complied with and dispose of it properly.

Specifically, you can refer to 9 CFR 94.5(c). To simplify (and, you know, consult a lawyer or something if you need legal advice on garbage for your airline), regulated garbage includes all "meals and other food that were available for consumption by passengers and crew on an aircraft but were not consumed" from international flights (except: Canada is excluded and Hawaii included). If you've got regulated garbage on your plane, it "may not be disposed of, placed on, or removed from" the plane unless it is disposed of properly (in this case, that's a fancy way of saying "very expensively"), in a way that ensures it won't introduce agricultural pests and disease. This even applies to leaving food on the plane for the return flight. "Passenger takes the food home and eats it later" is, sadly, not listed as an approved form of disposal.

So as I read the regulations, the crew was following the required procedure to ensure that all regulated garbage was collected for proper disposal.

While the other answers raise some good points, I think it is important to point out that technically the flight attendant is wrong. You are allowed to bring food into the USA, as long as you declare it to customs, and as long as it is not on the list of forbidden food products (which can change rapidly, for example in response to specific diseases/health hazards).

Many (most?) passengers do not declare any food. Bringing an undeclared sandwich can indeed result in fines and other unpleasant consequences. So, the flight attendant is giving some simplified advice to help you avoid these consequences. Declaring food (even food that is allowed) may delay your passage through customs, and may sometimes result in bags being opened, increased questioning, etc, so probably not worth it for a sandwich.

However, I often travel with lots of other food that I'm going to declare anyways. In situations like this, I've always just included food that I'm taking from the plane in my declaration, and never had any issues with it. The worst that can happen is that they'll throw it away.

djna is correct but not complete and it is very country specific "The USA (...)".

To answer your last question: no, it is a general rule, that you are not permitted to bring sandwiches outside airplane (so not just US stuffs), for few reasons:

• custom: food (also animals and plants) is very regulated in all countries, mostly because of possible disease (to agricultural sector).

• sanitary consideration: plane food is carefully controlled, safe and keep in cold, before to serve it to you, but after some hours on air temperature, the food could not be edible, and if you will have some problem, you will blame the airline. You should also consider that the food was not prepared on the airplane, so it is "relatively" old, so after is warmed up, it should be relatively eaten quickly.

For chocolates and snacks, usually the airlines are less severe. Such food is prepared (heated and sealed) so it should not be a concern for both the above points. Customs could not agree, but still safe to eat before you pass the custom. [tomatoes and cheese cannot be disinfected with heat]

Just like the EU, UAE, Australia, and USA all have a total ban on personal travellers carrying non-manufactured (i.e. Raw Agricultural) foods, including Dairy. The most serious offenses can result in jail time of up to 10 years! Also, if it has been proved that the Airline supplied the food, then they can also get into serious trouble with CBP.