Someone has already quoted the rule. A fielder may not block the runner's progress to the base unless the fielder has the ball. Home plate is no different, although as several have pointed out, it is not generally called by the umpires consistently with the way that the rule is written.
He's right. Blocking the base without the ball is interference and by rule the runner gets a free advance to the base.
HE'S WRONG. The rule only applies if there is no ball thrown to the base (in other words, no play) and any attempt to block passage of the baserunner is interference.
A defensive player has every right to position himself at whichever base the ball is being thrown to in order to make the out.......
By the rules of baseball, a runner has the right to an unobstructed path to a base. However, this right is not granted if the fielder guarding the base possesses the ball or is in the process of catching the ball.
The fielders guarding first through third base are unlikely to risk physical harm and will generally place themselves out of the path of the runner. The catcher guarding home plate, however, wears padding and a face mask and will frequently place his body as an obstacle between the runner and home plate. Since the runner does not have to worry about remaining on home plate, only tagging it, he will run at full speed in an effort to reach the final base. The speed of the runner combined with the fact that the catcher still has to tag him (unless the bases are loaded and the force play at home is still available) makes for a dramatic play.