Breeders may euthanize for a variety of reasons. Need for space, freezing to kill parasites, sparing the poor rattie from painful snake suffocation, sparing the snake from a fighting rat, etc. Bigger rats especially can do serious harm to a snake.
I picked up a dumbo female not long ago and I dread her first litter. I won't be able to feed off any of those fuzzy droopy-eared faces!