The word "open" may or may not be appropriate, but that's hardly the point.
Access to the WPBA US Open for anybody having the playing skills is actually considerable, and greater than ever before. In fact, the WPBA US Open has always awarded a spot to the winners (or highest non-pro finishers) of the numerous state championships contested on the regional WPBA tours throughout the calendar year.
There are many, many regional tours throughout the United States, and these regional tours have helped to jumpstart the pro careers of many.
It seems that each and every year, some players that go on to be high seeds on the WPBA tour "graduate" from the regional tour system. Testing the limits of my memory:
In 1999, it was Karen Corr and Monica Webb
In 2000, it was Julie Kelly
In 2001, it was Kim White
In 2002, it was Sarah Ellerby, Melissa Herndon and Kim Shaw
In 2003, it was Jennifer Barretta and Romana Dokovic
In 2004, it was Ga Young Kim
In 2005, it was Kelly Fisher
Every one of these players went on to a ranking of #12 or better on the WPBA tour. Five of them have been ranked #3 or better at some point.
If the WPBA US Open is not an "open" form the standpoint of the purist, it is certainly a tournament offering wide access to all worthy aspirants.