It's catch and release, not life long milking. Granted, the survival rate isn't as high as I'd like (70-90% apparently), but I do also appreciate having safe injectable medicines. All things considered, with a species bias, I'd prefer dozens of humans live at the expense of a.... Not crab. Unfortunate though it may be.
I can't also help but notice you've anthropomorphised them a bit. I'm certain these creatures respond to negative stimulus, but attributing fear and life long trauma seems to be giving their intelligence a bit of an unfounded boost.
Honestly, of all the messed up shit we do with animals draining blood from a bunch of crabs for medical purposes seems like one of the less egregious ones.