Because the odds of getting a double play in that case would be if the BR fails to or delays his/her advancement to 1B. The runners need to stay on or near the base or be subject to being doubled off. That possibility does not exist with the BR.

It's sorta covered w/ the intentionally dropped fly ball in the infield. Not 100% as an infielder can guide the ball to the ground [ASA].

To paraphrase what others have said:

The IFF rule is intended to protect the offense from a cheap double play due to runners that must stay at their base because the ball is obviously in flight and they would be doubled off if they run. With two or more such runners, it would be a fairly easy task by most players.

But nothing keeps that batter-runner from running, other than laziness or stupidity; there is certainly no jeopardy attached to the batter-runner running, like there is for the other runners. There is no reason to protect that person if he/she fails to run out their hit.

Thanks!!!!

If
 

the batter runs the play out then there shouldn't be any chance of getting a double play. But, if he's an idiot and tosses his bat down, mutters some obscenity, then sulks back to the dugout then he deserves to get doubled!