This is complete conjecture on my part, and I haven't rewatched it, but it seemed to me at the time that Spieth initially wanted to go up and mark his ball, but then stopped when he realized that Rose was already near Rose's ball. I thought both that Rose did that deliberately (to try and keep Spieth from marking) and that Spieth stopped because of the unwritten tour rule--that is, to avoid being labeled as the guy who would mark his ball in that situation only if it benefited him. Again, complete conjecture, but that's what went through my head.