Fed Rule 8
Baserunning
SECTION 1 WHEN BATTER BECOMES A RUNNER
ARTICLE 1. A batter becomes a runner with the right to attempt to score by advancing to first, second, third and home base in the listed order when:
c. an intentional base on balls is awarded, or a fourth ball is called by the umpire; [my emphasis]
A base on balls is based on the declaration of ball four, not a pitch passing the plate area. If it were based merely on the ball passing the plate area, it would totally negate the concept of the pitch being declared a ball or a strike by the umpire. It's HIS decision that counts regardless of where the pitch is.
The basic concept of interference is that an offensive player should not interfere with the defense's effort to retire a runner with exception of certain
needs of the offense. Needing to advance to 1B before ball four is called is not a
need allowed by rule.
At the time of the pitch when the runner broke for 2B he did not know if the pitch was a ball, strike, hit, foul, or HBP. At the time that F2 initiates his throw, it's quite possible he does not know if the pitch was a ball or a strike. That could be a reason for his play as opposed to the stupidity of playing upon a runner forced to 2nd due to a walk to the batter. Should he delay his play on advancing R1 because HE thought the pitch was a ball---regardless of what PU thinks? Should the runner start to 1B because HE thinks it was a ball---regardless of what the PU thinks? Even after declared a ball, there is still possibility R1 may come off the base after touching it. So, did the batter interfere with a play attempt before or after the base on balls? Quite possibly so.
While I'll not argue that Carl's answer is not unreasonable, it could only be supported by 10-2-3g regarding points not covered by the rules. However, an interference call is supported by the batter interference rule (at time of interference), and by rule 8-1-1c (shown above). A base on balls is not awarded until the umpire calls ball four. If interference occurred before that declaration, a ruling of batter interference is supported by rule. If interference is supported by rule, then 10-2-3g is not since the point
would be supported by rule.
While I am not necessarily advocating an interference call on the play described, I am saying it is not necessarily a wrong call. Certainly if R1 overslid 2B, an interference call could be a very viable potential---
supported by rules.
Just my opinion,
Freix