NFHS new Case Play

[Middleman]

*8.2.7 SITUATION B:

The leadoff hitter has a 3-2 count. There is a check-swing situation and the plate umpire calls ball four. The base umpire reverses the call on appeal and calls strike three. B1 overruns first base. In (a), the defense tags the batter/runner returning to first base. In (b, the defense tags the batter/runner after the batter/runner attempts to go to second base before returning to first base.

RULING: In (a), the batter/runner is safe. In (b), the batter/runner is out.

Without reading anything into the case, and without making any assumptions, and without adding anything that isn't there, can anyone tell me why in (a) the batter/runner is safe? I'd say he's out on strikes.