Refusal to thank one's teachers and dojomates is certainly rude, but I wouldn't put it up there with, say, injuring one's training partners.
Some say that wearing a white belt and behaving as a beginner when visiting other organizations is in fact more polite than claiming a rank that the other organization might not recognize.
Clearly there is more to this story than you have chosen to share. But without those additional details, others may not share your sense of outrage.
There are more details, but for various reasons I prefer not to divulge them. In this instance wearing a white belt is not a matter of choice, it is mandatory. Not telling the new teacher about your other dojo and affiliations is a matter of choice.
I don't believe that I am conveying a strong sense of outrage, just asking what experiences people have had. I am sure my story is fairly mild in the great scheme of things.