Because bosses won't even be able to drop the best items in the game, let alone good items. So you're *first* and only kill of any actual significant boss yields higher loot than normal, because well it was your toons first kill. After that it goes back to normal. It might sound exploitable, but making a toon just so you can level and kill a boss in hell or inferno (for anything good) wouldn't work out.
It's a reward for your first kill. Something to keep you going as you progress for the first time. The items only drop the lowest number of magical properties and the best stuff will still only drop randomly. Bosses will drop better loot once Nephalem Valor gets activated as well.
why is the beta?