Several scientific studies over the last few years have shown this to be false and freezing has not been considered a humane form of euthanasia for several years. The reptiles become torpid but they don't go to sleep, they feel themselves freeze which is not only very painful, but because they're cold they can't move. Unfortunately a lot of vets haven't gotten the word so the misinformation that freezing is humane continues to spread.
Euthanasia should be as quick and painless as possible for the animal. For a snake you're looking at pithing or totally destroying the brain. It's not surprising that people shy away from it - it's brutal, it can be messy, and it's emotionally difficult to destroy a life that you planned months or even years to create.