Why does Set-Acl on the drive root try to set ownership of the “object”?

I found the answer. Microsoft says

Unfortunately Get-Acl is missing some features. It always reads the full security descriptor even if you just want to modify the DACL. That’s why Set-ACL also wants to write the owner even if you have not changed it. Using the GetAccessControl method allows you to specify what part of the security descriptor you want to read.

Replace the Get-Acl call with

$acl = (Get-Item $path).GetAccessControl('Access')