I agree with Shorty and Bill...there are lots of little ways to cheat in golf.
While the majority of PGA Tour players likely play by the rules most of the time, when money is involved, a significant number of humans will cheat if they think they can get away with it. When I played JR golf tournaments, you would occasionally see a group of hacks all turn in low scores, yet none of these guys would ever go low when paired with legit players.
Marking closer to the hole makes no sense to me- too high a potential cost for very little reward, BUT, when the greens aren't perfect moving the ball fractionally to avoid a spike mark or imperfect lie might tempt some guys, especially when the cameras aren't around.
Players can also get a bit liberal on things like spike mark vs ball mark, relief from sprinkler heads/cart paths, entry points into hazards, not trying to drop exactly where they played from, etc. The smarter players who are trying to stretch things might even plead their case with a rules official to make sure they are covered.
If a ball moves slightly while a player is addressing it in the rough, I gotta think there are at least one or two guys out there that would not call it on themselves when nobody else saw it. Similar with a single leaf falling when a guy takes a practice swing.
It might not be cheating, but it seems like increasingly over the years I have noticed players taking advantage of dropping twice and then placing the ball. I have had to invoke this rule occasionally when there is a steep slope, but find it interesting how often Tour players utilize this on relatively flat ground.