A big part of moderation is enforcing what makes you (the moderator) uncomfortable. The moderator(s) probably have not come to terms with their views on eating meat, so the question makes them uncomfortable, so they ban the discussion to preserve their own feelings.

In a less selfish view, it also probably helps preserve the community because many others are also uncomfortable with their unresolved views on eating animals and might be discouraged from participating if it is discussed. You are probably a microminority in wanting to discuss it and the number of free speech absolutists who will die on the hill of your right to talk about eating roast mareep is also a microminority. As the discussion does not really add anything to the community it costs them almost nothing to keep people comfortable.