I've heard this before - its absolutely crazy that you, and others, draw a comparison between a mammal & plants, using a plants chemical response as justification for killing a living breathing mammal - you're just deflecting the uncomfortable truth of slitting a mammals throat and it then dying is the worst hell like event to live through (and then die shortly after), its quite literally as unpleasant as you imagine it would be if I did that to you.
Plants, as proven by science, don't have a "pain" response, they have a 'the plant is being attacked' response, its certainly not pain as you feel it.
As you're clearly not too up on science - let me help you, if it moves around, under its own power, it feels pain, if it grows via sunlight, entirely different, no real pain response - and yes crabs do feel pain, as do lobsters.
Where was this proven by science? I just read an article on Yahoo Answers that said that plants scream when you cut them.