It's not a question of "how do you decide which public standards should be freely accessible". That's easy: all of them. The public benefits more from freely accessible standards whether they're building codes, legal codes, ISO standards, or HDMI. The effect of not having them publicly available is that people make-do without having read the standard and the public has no way to validate things against the standard afterwards.

The question we don't have an easy answer for is how to incentivise the people behind these things without locking their work behind paywalls? Compliance marks, homologation regimes, copyright, and other strategies all have their own downsides.