I umpired baseball for six years and though I can be a bit rusty, this play is very clear.
1) The runner is under no obligation to provide the fielder with a 'fair chance' to throw to the target base.
Example A -- Catchers routinely field a ball directly in front of the plate. The batter running to first presents an obstacle to a straight line throw to first. So in tandem w the first baseman leaning the correct side, the catcher must usually take a single side step and throw a bit wide of the target.
Example B -- runner on first base w less than two out...sharp ground ball to first baseman fielded near the bag. If he wishes to throw to second for the force play (or step on first and throw to second for a tag play) the runner presents an obstacle which the first baseman must work around via a slightly wide throw one side or the other.
In both above examples, if the batted ball is fielded quick enough, the fielder may also have time to launch his throw over the top of the runner in a more direct line to the target.
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Secondly, as correctly noted last night's crew chief in New York, the runner is likewise not obligated to only run in the dirt portion of the line between home and first.
Truth be told, there is no "baseline" for the purpose of where a runner must run. He could - if so inclined, detour as many feet into foul or fair ground as he wishes as long as his body does not directly impede a fielder from fielding the batted ball. Of course he will take as straight a line as possible but if he swerves wide one way or the other without impeding the fielding of a batted ball he is good.
The common misconception from average viewer is that a runner "cannot leave the baseline".
In fact, he can run anywhere he wants with the exception being if a defender is attempting to tag him out. At that point if he veers away from the fielder holding the ball by more than 1.5 feet (judgement call) he is ruled Out without a tag needed.
Viewers perceive he was called out for "leaving the baseline", when in fact he was called out for physically avoiding a tag attempt by 1.5 feet or more
