Why would you be trying to find the point at which it is too loud? The goal is to amplify things to the point that you can hear them clearly, not to go beyond that.

Also hearing aids have manual volume control which a licensed professional has no influence over, so that doesn't really make sense as a justification for limiting availability.

It's certainly possible to apply volume limiting to headphones, though again the people using amplifying headphones typically are dealing with the opposite problem.