There are very few professions we’d consider this acceptable. Implicit in this statement is the assumption that your time-value is only known by you (or free).
You certainly wouldn’t let your contractor improve things on your dime, unplanned, and unexplained. You might even fire them if you discovered they spent the day “refactoring” conduit instead of installing the pot lights you asked them for.
Where you have a contractor hired on a full-time basis with the intent to built the best house, or at least the most moated house, on the market so that all the people of the world come to live in your house, not someone else's, of course you would.
In your fictional scenario of unlimited budget and time, sure I grant that an expert should work unguided.
Worsens what position?
> Worse, because they believe only they’re able to know what’s best.
Well, you certainly wouldn't hire a contractor if you knew better, would you? That would be pointless. The whole reason for hiring a contractor, instead of hiring the same laborers the contractor will go on to hire on you behalf anyway, is because the contractor brings the expertise you lack. If you can't trust them to know better than you, why bother? It just becomes an unnecessary expense and a waste of another person's time.
Your position is employers are building the best house with the biggest moat to attract the entire world, with an assumed endless time and budget, and there are no bounds to be set with employees.
If you thought your home builder was going to run over time and budget, you wouldn't hire him in the first place. Those who can't find the necessary trust don't build homes. There are plenty of used homes out there to buy.