Why do vegetarians think it's worse to kill mollusks(clams etc), than bugs, or eat dairy?

I'm not trolling, I know I asked a similar question earlier, but after getting some interesting answers I'd like to pose the question differently. I am a huge defender of animal rights, but recently discovered mollusks(clams, oysters etc) don't have brains. I realized also that I don't kill bugs, but they die for my veggies, I eat dairy and this is all ok with vegetarians, but when I mentioned eating mollusks, I actually got judged and "lost my vegetarian status" with some for eating clams. This seemed so hypcritical, are we missing the point? So I'm posing the question again. With all the talk about vegans not wanting to eat honey, how does it make sense that we don't insist on say hydroponics for our veggies, or start a humanely harvested "no bugs harmed when picked" veggie platform? If you are worried about the environment and the oceans, their are humane and sustainable options for mollusks, and if you eat seaweed, you can be sure fish are hurt and killed in the process as well.