I agree with most other people, that in most cases I think people are asking to NOT have an item they dislike--not thinking they can now get more of the rest. For the record, I don't generally eat tuna salad except that my mom makes, because I find people put all kinds of "weird" things in it that I don't like. I grew up with it being tuna, mayo, pickle relish, and hard-boiled egg. Sometimes we leave out the egg just because it's a pain to boil eggs. I think of these things more as "ingredients to make the food taste better," not fillers to make it stretch farther. I also dislike celery intensely and would rather have my lobster roll without it if that was an option--I wouldn't expect to get more lobster meat because of it.
In the roast example it sounds like people were being rude, if they took a lot more meat than their share. But, sometimes it's difficult to determine what your "share" is, and a lot of people don't even try (veteran of trying to order pizza for my office!). I think that's one of the hardest things about hosting, trying to figure out how much food you should have available for people, especially if you're serving an expensive specialty item. Honestly I think if you (generic) are deliberately using fillers to stretch a meal for your guests (barring extreme circumstances), you should rethink serving that particular food, or having guests at all. As a guest, I would try to take only my "share" of food, to avoid shorting other people or looking like a pig, but it would never occur to me to consider the host's finances as well. Another option is pre-dividing the food into the serving sizes you want and just giving that to people, rather than letting them decide.
It's debatable if the people were rude to ask the host to modify the meal at all, I think. It depends on the relationship
and the circumstances. I would ask my mom or best friend to leave out an ingredient I didn't like, for sure. Or if the host asked me, I would say I didn't like a certain thing. If I had an actual restriction/allergy/gag reaction, I would mention it. If it's just not liking something (like celery), I would probably just eat it (or pick it out once served). But again I think it usually has to do with not liking ingredient X, rather than thinking they're going to get more of ingredient Y now. And you don't have to give them more of ingredient Y, just give them the same amount you would have before, even if the overall amount of food is less.
And for the tuna salad stuffed in meatloaf, covered in chocolate pudding, or whatever it was...
Honestly if I liked all the component parts I would just eat them separately, and not eat the parts where they touched. If that offends the host and they don't want to invite me back... frankly I don't think I'd be too sad about that! Not that I would be deliberately trying to be rude, I would be trying to cheerfully make the best of the situation, but I'm not putting meatloaf and pudding in my mouth at the same time. I'm not a very adventurous eater but I don't have any actual restrictions/allergies, so if I'm just served something without a choice I'll try to make the best of it--eat the parts I can (and sometimes if I don't know what it is, I'll try it and find that I like it fine), fill up on sides, push the thing around on my plate, and hope no one quizzes me on it!