I posted this yesterday and got a reply that didn't solve anything, so
thought I'd try restating to see if a more revealing answer is available.

For testing, and to simplify the problem, I made a brand new file with no
other code. Rng_A is on an unprotected sheet, and Rng_B on the protected
one. The protection dialog box is checked for allowing user to select locked
cells. Of the examples below, the first works, while the second fails. It
seems to violate the intent of the UserInterfaceOnly setting. Anybody know
what's at the bottom of this?

Example 1:
Sheets(1).unprotect
Range("Rng_A").copy Range("Rng_B")

Example 2:
Sheets(1).protect UserInterfaceOnly:=True
Range("Rng_A").copy Range("Rng_B")