SCOTUS Is Right. Your Understanding of Administrative Law Is Wrong
forbes.com
forbes.com
It's still bullshit. It's just different bullshit from the bullshit you're talking about.
For this Court, the Legislative branch is meant to write laws, the Executive branch is meant to enforce them, and the Judicial branch is meant to interpret them. They've very consistently rolled back cases where the Judicial or Executive overstepped and started writing laws on their own.
So as for the why: they really seem to sincerely believe that government will work better when the branches stay in their lanes.