[UmpJM]

Bob,

You said:

Not true. Only the runners need advance. ...

But that's not what the rules say. And this can't be one of those FED differences because under FED the ball is immediately dead - so nobody can advance anywhere on play following tha balk. What am I missing?

From 8.05 (my emphasis):

PENALTY: The ball is dead, and each runner shall advance one base without liability to be put out, unless the batter reaches first on a hit, an error, a base on balls, a hit batter, or otherwise, and all other runners advance at least one base, in which case the play proceeds without reference to the balk.

From the MLBUM (my emphasis:

7.8 PENALTY FOR BALK

Under Official Baseball Rule 8.05, the penalty for a balk shall be: The ball is dead (when play stops), and each runner shall advance one base without liability to be put out unless the batter reaches first base on a hit, an error, a base on balls, a hit batter, or otherwise and all other runners advance at least one base, in which case the play proceeds without reference to the balk.

The penalty for a balk provides that if a batter reaches first base safely on a hit or error, base on balls, or otherwise on a pitch on which a balk is called, he shall be entitled to first base only if all other runners have advanced one base or more on the play, in which case the balk is disregarded.

If the batter-runner and all other runners do not advance, the balk penalty prevails, the batter must return to the batter's box with the previous count, and runners advance one base as penalty for the balk.

JM