" UNSPORTING CONDUCT
Players are expected to behave in a mature and considerate manner and to play within the rules and not abuse them. This prohibits intentionally stalling a game for time, placing components with excessive force, inappropriate behavior, treating an opponent with a lack of courtesy or respect, cheating, etc. Collusion among players to manipulate scoring is expressly forbidden.
The organizer, at his or her sole discretion, may remove players from the tournament for unsporting conduct. "
-Star Wars Destiny Tournament Regulations, Page 3
There. Someone quotes the rules on the matter. It is unquestionably cheating and abuse of the rules to not randomly generate a number (die face) using a random number generator (die) when instructed to do so (roll).
Can a player or judge "prove" an intent to cheat, beyond any shadow of a doubt? No. Do they have to? No, this isn't a court of law trying to convict someone of murder, this is a tournament for a game. They don't need to set as high of standards to demonstrate guilt, they can set them as low as "because the judge thinks so" and that is good enough.