Why you can't upgrade if you're vegetarian

As told to me by a pal who recently flew out of Heathrow with a famous national carrier... arrived nice and early to make sure upgrades were available... here's what happened (paraphrased by YT):

CHECK IN:

"Would you be interested in upgrading to business class?"

PAL:

"Yeah, why not...?"

CHECK IN:

"Brilliant. Give me a moment while I just sort that out for you..."

(CHECK IN TAPS AWAY AT KEYBOARD)

CHECK IN (NONPLUSSED):

"...it says here that you've booked a vegetarian meal..."

PAL:

"Naturally, being a vegetarian means one generally does book a vegetarian meal."

CHECK IN:

"I'm sorry, but you won't be able to upgrade."

PAL:

"WHAT!!!"

CHECK IN:

"I'm sorry. There is a space for you..."

PAL:

"Then what's the problem?"

CHECK IN:

"It's just that if we upgrade you to business class, someone in standard class will be forced to have the vegetarian meal you pre-booked. Sorry about that. Enjoy your flight. Next!"

PAL:

"What about if I just have the standard class meal?"

CHECK IN:

"Can't be done, Sir. The meals in business and standard are served by different staff from different galleys. Have a nice day."