Why do you guys all keep harping on this purported unenforceability?
Because I suspect that enforceability may be part of the reason of why the USGA has so far refused to ban the practice.
Enforceability becomes a issue when a competitor calls a rules official to come over and determine whether a trademark, logo, shape, series of dots, seam, or anything else about his opponent's ball really does indicate a line for putting or not. And because there is no hard-and-fast rule for him to determine that, he'll have to rely on his own judgment, so that enforcement of the rule depends on the judgment of individual rules officials. Remember the stink caused by Ernie Els at the Masters a few years ago when he kept calling rules officials over until he found one that let him take relief from brush that had been "piled for removal"? My guess is that the USGA wants to avoid introducing new rules that rely too much on the judgement of individual rules officials and therefore will not be applicable across the board.
On the other hand, when a player tees off in front of the markers, there is a clear method for an official to determine whether the ball was in front of the markers or not (in other words, he knows what questions to ask). Same for determining whether a ball is OB or not. Same with a guy bumping his ball, illegally substituting a ball, playing out of turn in match play, catching sand on his takeaway in a bunker, knocking down leaves or pine needles in a practice swing, and most if not all of the other rules situations you have cited as similar in enforceability.
That's why about 1000 pages ago I challenged you to write a hard-and-fast rule for helping a rules official determine if a mark really indicates the line for putting or not. Something like "A mark on a golf ball is deemed to indicate a line for putting if, when the player replaces his ball on the green, the mark has the following characteristics: _________________" that any rules official anywhere can use to arrive at the same interpretation of whatever mark they see on a ball and however the ball is oriented on the green. If I remember, you made a pretty good attempt at it, and indeed I think if you ever want to see the rule changed, then this is your key challenge.
But again, that's just my guess at what the issue is--I have asked the USGA about all this and will report what they reply if and when they do.