The reason the balk advances the runners regardless of whether they are 'forced' to advance or not is that a balk is called to protect the runners from the pitcher using deception to cause them to leave their base early, and thus be picked off. As such, when a balk is called, the game situation is treated as if the runners had been stealing a base, and they are awarded one base. This goes a bit beyond a 'return to equity' of course, as often no runners are going when a balk is called, but that is the general reason.
From the comment on the specific rule:
Comment: Umpires should bear in mind that the purpose of the balk rule is to prevent the pitcher from deliberately deceiving the base runner.
If there is doubt in the umpires mind, the intent of the pitcher should govern.
Additionally, a balk never results in a ball to the batter. Instead, there are a few possible situations where a ball to the batter might be given instead of a balk, or in addition to a balk; but the term
balk specifically applies only to the advancement of the runners. Note the comment to 8.05e (the Quick Pitch rule):
With runners on base the penalty is a balk; with no runners on base, it is a ball.
And 8.02a (the Doctored Ball rule referred to by diggers), when a play occurs and the manager does not elect to accept the play's results:
(d) If the manager of the team at bat does not elect to accept the play, the umpire-inchief shall call an automatic ball and, if there are any runners on base, a balk.
In both cases, the balk and the ball are clearly separate items.
Also note that if a balk and a walk occur simultaneously - as could occur in either of the two above situations - any runners who would be 'forced' by the walk do not gain an additional base.