My general rule is, if I don't get a reply from a model, then I was not fit for the casting. If I don't respond to a model who has applied for my casting, then she was not what I was looking for. I kinda thought that was the accepted norm - without being rude.
Sometimes photographers get inundated with such a vast amount of replies it is impossible to message everyone back individually to let them know that they haven't got it. I have been modelling for years now (only just recently on purestorm) and it's always the case if you don't hear you were unsuccessful. It's not rude. Just practical.
I works both ways, but I agree with you, its just rude. It takes little time to open a message and read it and it takes little time to reply 'Thank you for your interest, unfortunately you are not suitable for this call' or something like that.
Just move on. Why waste your time with someone that doesn't have the courtesy to send a quick reply.