If it wasn't the ADA, then it could have been another disability law that applies to ASA as a part of USA Softball, which gets funding from the USOC which gets funding from Congress. There's connections in which someone could have sued using one of the disability laws. I distinctly remember there was a person from Nebraska who sued the ASA because the rules didn't allow for her to play.

If these rules we're discussing were enacted before 1990 (and I don't remember them from back then), then they did predate the ADA.