What's the reasoning?

[greymule]

I think that the rule was written because some coach or coaches had actually tried to implement the play.

Without the specific rule, you might have one out for one runner passing the other. We also have the rule that puts a batter or runner out for USC (BR throws bat in anger, ball goes over fence, BR still called out; or runner slides safely into 3B but throws an elbow into F5's face: runner out). If each runner has committed USC, they both could be called out.

On the other hand, the rule applies the outs even if the runners switch unintentionally. It would be tough to characterize an unintentional mistake as USC.