Improperly tuned, they will damage your hearing - a permanent effect.
I fail to see how adequate directions for use could not be prepared.
Do you have the tools to measure how loud your headphones are? I don't. And so I keep them as low as I can.
Is there a limiter in amplifying headphones to keep them from going above a certain threshold (like 80Db)?
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.
hearing loss is far more than just "make it louder".
Hearing aids can exceed 120dB.
As well improper mid-long term use can cause comprehension and "hearing in difficult situations" issues.