Ecorazzi notes that Anthony Bourdain uses the argument that vegetarians/vegans are bad because they are bad guests. Bourdain himself:
“They make for bad travelers and bad guests. [...] you’re unwilling to try things that people take so personally and are so proud of and so generous with, I don’t understand that, and I think it’s rude. You’re at Grandma’s house, you eat what Grandma serves you.”
It's a bit annoying that one even needs to refute the "You're bad if you don't betray your morals for the sake of a host's feelings" argument (after all, shouldn't "hosts" be at least as concerned about their "guests"?). Certainly one could come up with absurd hypothetical examples of behavior no guest would be expected to engage in out of politeness. But it seems that food comes with a whole different set of rules when it comes to discussion of both ethics and hospitality. Food is intimately tied up in hospitality, and behavior around food is central to a host-guest relationship. There are all sorts of social customs, even rules, about it. But we don't have to invent outrageous hypotheticals to show how silly this line of argument still is. In fact, we can turn to another central behavior of hospitality, of the expected relationship between hosts and guests: conversation.
Talking is a regular part of hospitality. Hosts and guests chat, sometimes engaging in small talk, sometimes discussing current events, sometimes catching up on each others' lives, sometimes even just trying to amuse each other. That's common and expected, and there's a certain expectation of politeness surrounding the conversation.
But let us say that you are a guest, and your host begins telling racist jokes. Would it be rude not to laugh? Would it be rude to tell the host that you don't like racist jokes? Furthermore, should you care if it is rude? Would you say "Well, I'm at Grandma's house, so I have to talk about what Grandma decides we'll talk about?" Maybe an otherwise hospitable host telling racist jokes makes for an awkward, uncomfortable moment. Maybe it will be a strain one way or another no matter how you decide to handle it. But would you really say that one is "rude," a "bad guest" if he or she didn't want to engage in racism? And would you really put the burden of rudeness on the guest for this situation?
Of course not. But this is the sort of logic that happens around eating animals, because people often have such wildly different ideas of what it means, or whether it matters at all, to be eating animals. Because food is necessary, everyday, social, and personal, we have whole different rules of logic about it. And for some who focus a great deal of attention on the eating of food but who have no regard for an animal as a creature deserving of ethical treatment, it will of course be a greater sin to offend a host (even if you politely decline!) than to eat an animal for your own pleasure. But those same people wouldn't expect their logic about food to be applied to similar situations.