The reason kittens should use non clumping litter is it makes the kitten have a hard time have ingesting the cat litter.
Young kittens are very curious and may try eating the clumping cat litter which can cause problems with the cat which could potentially be fatal so that's why it's recommended that kittens use non clumping cat litter so they won't be able to ingest it as easy as the clumping cat litter.
Ingesting clumping cat litter can cause gastrointestinal upset or even an intestinal blockage in your kitten which is not good.