Tagging a runner

It all comes down to the definition of a tag.  Emphasis mine.  Explaining why the rules are written that way is just speculation. 

TAG is the action of a fielder in touching a base with the body while holding the ball securely and firmly in the hand or glove; or touching a runner with the ball or with the hand or glove holding the ball, while holding the ball securely and firmly in the hand or glove. It is not a tag, however, if simultaneously or immediately following his touching a base or touching a runner, the fielder drops the ball. In establishing the validity of the tag, the fielder shall hold the ball long enough to prove that he has complete control of the ball. If the fielder has made a tag and drops the ball while in the act of making a throw following the tag, the tag shall be adjudged to have been made. For purposes of this definition any jewelry being worn by a player (e.g., necklaces, bracelets, etc.) shall not constitute a part of the player’s body.

Thanks for answering, I know it is not really a "rules" interpretation type of question but it really made me wonder about the "why" aspect of the decision process.  The "hand or glove" is written into the rule, and really no way to discern the "why" it must be that way, just seems inconsistent that the base is treated different than a player, although I can see why being able to touch a player with a cleat or hand could be dangerous.

To be consistent it seems you should have to touch the base with the glove/ball?  Of course that would make double plays pretty impossible, but would be consistent. 

So the fielder with the ball doesn't slap or otherwise beat the runner with his free hand as he goes by - ?

There is also a requirement for a tag to be considered an out, the defensive player must retain control of the ball through the tag.  It would be nearly impossible to dislodge the ball if the tag were made with a body part that did not control the ball.