CLUSTER, REINDEX, VACUUM in "read only" transaction?

So I was looking for other omissions in utility.c, and I noticed that
check_xact_readonly() doesn't reject CLUSTER, REINDEX, or VACUUM.
Now the notion of "read only" that we're trying to enforce is pretty
weak (I think it's effectively "no writes to non-temp tables").
But I can't see that CLUSTER is a read-only operation even under the
weakest definitions, and I'm not seeing the rationale for REINDEX or
VACUUM here either.

Comments?
        regards, tom lane