It's a blanket limitation on HUM (Highly Unusual Method) systems employing a Forcing Pass or the like; by disallowing a catch-all substitution bid in place of an opening Pass. The effect is that such systems must, systemically, describe for the opponents how every hand type enters the auction.
The greatest virtue of HUM systems is the unfamiliarity of it to the opponents. This regulation combats that.
Further, I suspect it's a dig at proponents of EHAA (Every Hand An Adventure) type systems who are claiming non-agreement and non-existence of agreements on bidding patterns. Of course they built experience, and experiential agreements; now it's in black-and-white that those must be provided, in black-and-white, for the opponents in a timely manner that allows development of at least modest familiarity.
As a final note, your comment that players of such systems automatically lose: that's false. Look up the history of EHAA and Forcing Pass systms going back to the early '70's and check it out. What they actually did was undermine the skill component of winning, randomizing the results unacceptably.