Our laws of science tell us gravity is constant, yet there are many spells that change this. Given that lycanthropy curse is a magical efect, I fail to see how that argument holds.
@SevenSidedDie you are free to agree or disagree as you wish. Why then would a werewolves clothes be destroyed but not a wererat or wereboar? The art shows them wearing gear, based on your apparent belief their original gear - they are medium bipedal creatures. Why is the wolf'storn but the others intact?
The question's asker based his question on the inconsistency of the visual representation. I propose that the visual representation is meaningless. The wereboar and wererat are wearing clothes because they want to, and so could the werebear; he would simply need clothes made for a large creature.
But that werebear will eventually revert to his human form and that gear fit fora large creature will be useless. So he chooses not to wear any.
Because when he reverts, he will have his original gear, untouched because it melded into his body as per the description of Polymorph.