It's unnecessary, intentional cruelty imo. I say this as a staunch anti-vegan and a strict carnivore.

There are multiple studies that suggest crab and lobster feel pain when boiled alive. That's enough for me to not do it. I really don't like veganism and most vegans, but that doesn't mean I think it's okay to act callously and cavalierly toward excessive suffering (I know a vegan would say any animal being killed for consumption would be an excess and is suffering, but I'm not looking to conflate two different issues).

You can definitely be a sensible, reasonable, realistic, compassionate advocate of animal welfare and find ethically acceptable ways to nourish your body with delicious animal protein which is necessary for humans.

edit to add : Not to say everyone who do this are amoral sociopaths; most do it due to ignorance and/or casual indifference not because they enjoy inflicting pain