Simply put, the Runner is outnumbered. Cut the Runner a break.
Let’s put a finer point on it. In contract law, the law of adhesion states that when one party sets all parameters of a given contract with no chance for negotiation, the second party always wins any arbitration having to do with vague language or questions left open to interpretation. The first party must adhere to terms expressly spelled out in the contract.
The Pitcher and the Defense are the first party in our baseball scenario. The terms of each half-inning (the contract) are clearly in favor of the first party, which is why we get excited when the Batter, the second party in this case, hits safely a mere one third of the time. Just as a second party comes to a contract at a disadvantage when, say, buying insurance, so to the Batter steps into the box knowing he or she is outnumbered 9 to 1 and that the odds of hitting safely are historically slim (see also the law of large numbers). Applying the law of adhesion requires that in the case of a Batter/Runner reaching a given base and the ball reaching that same base via delivery from one Defensive Player to another (assuming the second Defensive Player is touching the base in question) at the same time, the judgement ought to go to the Batter/Runner as the party with less ability to negotiate more favorable terms.
Like the law of adhesion, the law of utmost good faith (all parties to a contract are assumed to be honest) is only applicable in baseball in certain situations. PEDs are banned, as are certain pitches, substances, and practices. But a catcher trying to deceive an umpire after a pitch that misses, an outfielder taking credit for catching a fly ball when he/she really only trapped it, or any number of other instances of gamesmanship are not only permitted, but also widely lauded. Gamesmanship is an integral part of the psychology and cerebral appeal of baseball. It’s why traditional fans want nothing to do with expanded instant replay.
Applying the law of adhesion to apparent ties is likely the best and only way to resolve MLB’s vexing (or, if you’re like me, charming) lack of clarity on the issue.