Dave,
To understand this rule you have to look at all of the rule. Wording in 8-2a helps to explain 8-2b.
8-2. Indicating Line of Play
a. Other Than On Putting Green
Except on the putting green, a player may have the line of play indicated to him by anyone, but no one shall be positioned by the player on or close to the line or an extension of the line beyond the hole while the stroke is being played. Any mark placed during the play of a hole by the player or with his knowledge to indicate the line shall be removed before the stroke is played.
b. On the Putting Green
When the player’s ball is on the putting green, the player, his partner or either of their caddies may, before but not during the stroke, point out a line for putting, but in so doing the putting green shall not be touched. No mark shall be placed anywhere to indicate a line for putting.
It seems an easy progression of thought to understand that "A player or caddie may, before but not during the stroke, point out a line for putting, but in so doing the putting green shall not be touched" and "No mark shall be placed anywhere to indicate a line for putting" has nothing to do with the ball or any equipment of the player's. It only relates to physically touching the putting green in an area that may be construed as "The Line".
Using the premise that all shots in golf are to targets, and it is these target lines that are the subject of this rule, might help you to understand why it is that no person or thing is allowed to remain on these lines during a shot. A line or mark on equipment or balls is not going to aid a player in hitting the ball on the target line, therefore no rule against them.
If this were not the case you could melt down all the two-ball putters and take the < off the crown of your driver.