In the 1988 rules of the PBA (US pro tour) Rule 4.9 said:
Players may touch object balls only to assist the referee in his duties. If a player intentionally touches any other object ball, for any other purpose, while a game is in progress, whether that object ball is in play or not, he has fouled. (This rule does not apply to the usual collisions between the balls.)
For an edition or two prior to 1992, the BCA Rule Book had both BCA and PBTA (new name) rules for 9 ball. In the 1992 BCA Rule Book, the BCA gave up it's own set of nine ball rules, and had only the PBTA rules, with Rule 4.9 identical to the above. The PBTA rules were stand-alone and not an integrated part of the BCA rules for such things as definitions. This was around the time the WPA was forming.
In the 1993 edition of the BCA Rule Book, the rules of 9 ball were pruned to remove the general rules so they only had rules directly about 9-ball. This was done to make one, uniform set of rules for pool and have a set of "World Standardized Rules." The idea of a player being allowed to touch out-of-play object balls was removed. I don't know whether it was by accident or intentional.
In the 2006/2008 WPA major rules rewrite, I did not notice that the rule was gone or I would have tried to reinsert it. I think it has to go back in, perhaps in a slightly different form. I'm a little prejudiced in this, since I wrote the PBA rules.
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