As for the reason it's rude, you are there as a group. Just because you don't think you need a tank doesn't mean you get to run off and start doing the dungeon yourself. Want to solo the dungeon? Go run into the instance, and do it yourself.
When the tank que'd for a dungeon, he did so with the reasonable expectation of actually getting to tank and perform his role.
Edit: The amount of people who don't understand the basic principles of respect is overwhelmingly sad.
This is what happens when parents don't know how to actually raise a child.