One issue is semantic - US provisional applications are not considered published but they are made available to the public by the USPTO to see when a corresponding non-provisional application or issued patent is published.
iv) Unpublished abandoned applications (including provisional
applications) that are identified or relied upon. The file contents of
an unpublished, abandoned application may be made available to the
public if the application is identified in a U.S. patent, a statutory
invention registration, a U.S. patent application publication, an
international publication of an international application under PCT
Article 21(2), or a publication of an international registration under
Hague Agreement Article 10(3) of an international design application
designating the United States. An application is considered to have
been identified in a document, such as a patent, when the application
number or serial number and filing date, first named inventor, title,
and filing date or other application specific information are provided
in the text of the patent, but not when the same identification is
made in a paper in the file contents of the patent and is not included
in the printed patent.
Separately, the PCT process requires the applicant to file a publicly viewable copy of any application relied on for a priority claim. It must be a copy certified by the original office it was filed with.
Any other set of rules would leave the world (including other patent offices) unable to judge the value of the priority claim. Is some purported prior art knocked out by the content of the provisional or not?
Regulations under the PCT
The Priority Document
17.1 Obligation to Submit Copy of Earlier National or International Application
(a) Where the priority of an earlier national or international application is claimed under Article 8, a copy of that earlier application, certified by the authority with which it was filed (“the priority document”), shall, unless that priority document has already been filed with the receiving Office together with the international application in which the priority claim is made, and subject to paragraphs (b) and (b-bis), be submitted by the applicant to the International Bureau or to the receiving Office not later than 16 months after the priority date,