Puzzle: Creating an INDEX requires EXCLUSIVE use of the table, doesn't it?

A coworker asked a question about some code. It creates a table with 2 fields, reopens the table SHARED, creates an index tag using the INDEX command, closes all, then tries to do the same thing, but this time it fails. Why?

CLEAR ALL

CLEAR

SET EXCLUSIVE OFF

CLOSE DATABASES all

CREATE TABLE foo (name1 c(10), name2 c(10))

TRY

      USE foo SHARED && reopen the table shared

      ?"Creating index with exclusive=",SET("Exclusive")

      INDEX on name1 TAG name1

      ?"Success creating index"

      CLOSE DATABASES all

      USE foo SHARED && change this to EXCLUSIVE to avoid the error

      ?"Creating index with exclusive=",SET("Exclusive")

      INDEX on name2 TAG name2

CATCH TO oMsg

      ?"Error:",oMsg.Message,oMsg.details,oMsg.LineContents

ENDTRY

This code produces the output:

Creating index with exclusive= OFF

Success creating index

Creating index with exclusive= OFF

Error: File must be opened exclusively. INDEX on name2 TAG name2

Is this a bug? Can you explain the behavior?