Question

Why did white settlers disregard treaty terms

Answer 1

Because white people are evil, no because frankly they just did not care.  They felt that their parents were born here before the Revolutionary war, and that they had a right, Manifest Destiny, to spread however far West they should please.  They spread all the way.  They regarded the Natives as uncivilized murders and rapists.  They were primarily ethnocentric and narcissistic.