I've heard a theory that I find semi-convincing, that John Twist is not only grief stricken but that the reason that he denies Ennis the ashes is because he doesn't feel Ennis "deserves" them. Because Ennis rejected Jack's offer and let Jack down, OMT feels he shouldn't get family member status and respect, and is unworthy of scattering the ashes.
On further reflection, I think the trouble I have with finding this theory even "semi-convincing" is that:
a) If by Ennis rejecting Jack's offer you mean the "proposal" that he and Ennis ranch up together, I don't think we have any evidence to know that John Twist knew that Jack had made that offer to Ennis; and
b) While we know that Jack talked to his father about bringing Ennis up to Lightning Flat, I don't think we have any direct evidence that Jack ever asked Ennis to go up to Lightning Flat with him.
We have to be careful not to confuse what we, from our position of omniscience
, know about the characters, and what they say and do, with what the characters know of each other's statements and actions.