Law is a technical language, just like medicine or physics. I certainly don't call it a virtue that it can be impenetrable to non-lawyers. But impenetrability is a different critique than the ones you are highlighting.
A lawyer who thinks phrases like:
"fair use"
"free, appropriate, public education"
"malice aforethought"
"you may have the body" (habeas corpus)
are self-defining is an idiot. My only point was that I didn't think the judge was making that mistake.
The problem here isn't opacity, as in "you have the body". The issue with "best interest" in the sense that I understand you to be describing is that it's misleading, and at the same time a more sensible alternative is readily available. A judge could instead say
It is strange, but nevertheless true, that even the freedom to make unwise decisions is one that the court is required to guard and only to restrict if and when the person is incompetent to make decisions.”
Note that the word "incompetent" as used here would itself ...
Autistic woman banned from having sex in latest Court of Protection case
This reminded me of previous LW comments about how we restrict the rights of children for their own good.
On the one hand, children can't understand the risks so we stop them having sex.
But on the other hand, animals can't understand the risks and we happily let them continue having sex.