You seem to be saying that "X is in the best interest of Y" is simply legal jargon for "X is in the best interest of Y and Y is legally incompetent to make decisions". If so, then this is a case of terrible terminology.
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.
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.