I was going to end the last post on this but it got long and it's a completely different topic anyway.
FJB provides one side of the catcher blocking the plate issue. The other? Fielders are not allowed to impede runners between the batters box and home because they could potentially move on from that base to the next one. Blocking forward progress at those points could change how many bases a player takes and therefore how the game is played. Home plate is a destination. There is no change in the overall natural flow of the game created when someone blocks home plate.
Given the already protected nature of the catcher (far more gear that any other position) and equipment geared specifically toward being a stable target rather than a moving one (Ever try to swipe tag with a catcher's mitt?) it makes sense that the catcher block the plate. While I haven't given much thought either way to whether it should be allowed or not there is nothing unnatural or outside the scope of the game about this play.