Straight from the KM May 1999, pages 3-6 article entitled: "1999 'God's Prophetic Word' District Conventions":
16
In the interests of time and simplicity, the Society has asked us to bring our own lunches to the convention each day. Most brothers have followed this direction and have found that after being dismissed for the noon break, they can sit down with their families and eat what they have brought along for the day. Many have said that the opportunity to relax during the noon break and spend additional time with their brothers and sisters has been enjoyable. This requires purchasing foodstuffs and beverages ahead of time and packing them in a small container or cooler that can fit under the seat. We request that all in attendance follow this instruction. Lately, building managers and food vendors in the facilities where our conventions are held have observed some brothers leaving the auditorium to purchase food from street vendors and fast-food establishments nearby. Some of the food vendors in the convention facilities being used are now demanding that they be allowed to open their food stands and serve the delegates attending our conventions. If this were permitted, many worldly people would be mixing in company with us during the day. Also, congestion and long lines around the concession stands would return, disturbing the relaxed atmosphere of our conventions. Therefore, we ask that each one endeavor to bring his own food and beverage to the convention each day. Interested ones accompanying you to the convention should also bring their own food. Glass containers and alcoholic beverages are not permitted in the convention facilities.
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I remember sitting through this part (and each part over the next few years talking about convention behaviour) thinking how easy it would be to leave during lunch and go to a vendor anyway. At our venue, no one would know because it is several blocks from any restaurants. Well, only the other dubbies that broke the rule would know, anyway.
Anyway, the above quote is the original source of the rule on bringing lunch. It is reiterated again and again in different km's over the years.
km 4/03 pg 3:
4
It is requested that all bring a lunch rather than leave the convention site to obtain food during the noon break. Your cooperation with this arrangement contributes to a peaceful atmosphere and allows more time for association with fellow believers. (Ps. 133:1-3) Please bear in mind that glass containers and alcoholic beverages are not permitted in the convention facilities.
km 4/05 pg 4-5:
7
It is requested that we bring a lunch rather than leave the convention site to obtain food during the noon break. It was commendable that most delegates carefully followed this direction at last year’s convention. How good it would be if all would do so this year! (Heb. 13:17) This arrangement provides a fine opportunity to enjoy upbuilding fellowship with our brothers and contributes to an atmosphere of unity and peace that brings honor to Jehovah.—Ps. 133:1.
Heh, this is the most research into WT literature that I've done in a year, I think.