I am surprised that any animal welfare organization would encourage in any way the boiling alive of living creatures. It's incompatible with respect for life. I'll ignore for now whether the Five Freedoms the SPCA promotes should apply to invertebrates, especially after they've beeen caught and are being "cared for" by humans. I don't know the SPCA official policy. Maybe somebody could speak to that.
For people who will argue that crabs have no nervous systems, an argument I've heard before, I'd like to say that they are dead wrong.