I see several reasons people become upset about others’ food choices.
First, food is the basic trigger of our disgust instinct. It protects us from eating things we don’t know are safely edible. We learn what is food and what is not very early in our lives and it’s hard to change. Some people can overcome that instinct better than others, but its still there in all of us.
Second, food is a tribal marker. Our in-group eats this. That out-groups eat that. We can improve our status in the in-group by disparaging the out-group. Or, if we’re surrounded by outsiders, we fear being mocked. Interlopers are inherently threatening to us, even when we try to compensate intellectually.
Third, people are authoritarian. We know what is right and what is wrong. There’s a right way to eat food and many wrong ways. We will enforce our norms, because people should be good and not bad.
So when we see someone eating something we would not, it can trigger visceral disgust, fear of the other, and moral indignation, at varying levels depending on circumstance. Hit the trifecta and no wonder we get angry.