Slater is obviously the ultimate competitor, and the rules dictate that this move was well within his "priority" rights. But I think the understood and universally accepted surfing etiquette should carry over to contest priority rules in this situation. When Parko was already driving through a perfect tube like that, the wave should be his regardless of priority. Maybe the rule should apply only at take-off? If he rode that through it would have been the wave of the contest.
Glad to see Slater win though. The still shot of Parko flipping him off from the shoulder is classic!