The theory of case behind the "no business-class seat without a business-class meal" policy is two-fold:

1). Airlines don't really want to call attention to UGs. The Y-class meal in front of someone makes it immediately obvious that he/she was upgraded.

2). Protection of the product reputation. In the Airline's view, the J-class product is a holistic experience---expensive, and one where the "value-for-money" equation is not 100% obvious. Reputation matters, so the last thing you want is someone reporting on the product when the experience had obvious defects ("I flew DL BE last week....wasn't at all impressed with the food"). Now, those of us who fly Biz-class regularly know that this second fear is a bit crazy (the food is bad anyway), but you get the point.

Bottom-line, airlines take this seriously. I was once flying AF ATL-CDG on the late PM departure. I was booked in J-class, but this was the first leg of a 4-consecutive-nights-on-planes-trip, so I asked to UG to F-class (with $$). The station manager said he'd be glad to do it, but that he needed to look into the availability of catering, as F-class was booked to zero. I told him I wasn't going to eat ANYTHING (I go straight to sleep on these short flights). He said this was a question of policy...I said I understood. He called me back a few minutes later and said that catering could be arranged, so he was happy to take my money ($1500 or so). As I handed over my credit-card, I thanked him for making this possible and pleaded with him not to cater the F-cabin (for those of you who don't know, F-cabin catering is done is blocks of six-meals at a time). Not possible, he said....a non-negotiable policy. So the cabin was catered with at least one of everything on the F-class menu, and, as promised, I went to sleep.