[officialtony]

It was a correct call
The NFHS rule is the only one I could offer, but I know there are others out there who can cite MLB rule.
A base on balls entitles a runner to adavnce to a base. It does not, however, protect him from stepping off the base once he has acquired the base and the ball is still live. He is only protected advancing to the base and staying on it. By sliding into and past the base ( or off of it ) he has, by rule, acquired the base and is now eligible to be tagged if he is off the base. The NFHS rule that covers this ( I think ) is 8-2-4h and note the exception. Although this rule states the batter- runner, I believe it applies to all runners at the time of the base on balls.
Please correct me if this is incorrect information.
My humble opinion.