In over 100 tournaments, I have never called anyone for this rule. Like I said, if I saw someone throw something down on the ground a few feet in front of their lie, I would ask them to remove it the first time, 2nd time...call them out. And here's why.....since a few of you don't understand how it could possibly be a visual aid.
When it comes to putting and approach shots, the window needed to hit your line is typically only a few feet in front of you. Once you hit that window, the rest is up to the disc. Granted it's not on the ground, it's a window probably anywhere from 5ft-7ft off the ground, but only a few feet in front of you. Distance perception is vital in this game, we all look for visual clues to get an idea how far the disc is to the basket. Especially on fairways with few features, in this case a rock, broken branch, etc, could be very helpful in getting a feel for distance and hitting a line. Most of us will do this in a subliminal nature and not even realize we're doing so. So sure, in the rare case when a player obviously places something "in front" of their marker, it's going to get my attention.