What?
Putting a live mammal in the freezer is not humane in the slightest, whoever is doing that is possibly breaking the law. No respectable vet, breeder, snake enthusiast or otherwise would even do or entertain the thought of such a thing. I don't know where you're getting this information from.
Cervical dislocation and CO2 are the methods used which are approved by vets and animal laboratories the world over. Whoever chucks them alive in a freezer is just committing animal cruelty.