Greg from Bandai America explained this to Power Rangers Talk at NYCC.

Basically, one of Bandai America's top sellers year after year are the morpher and the large deluxe roleplay weapon. In most cases that's a morpher (Samurai Morpher, Megaforce Morpher, Super Megaforce Morpher, Dino Charge Morpher) and a sword (Samurai sword, Megaforce Ultra weapons, Super Megaforce Sabers, Dino Charge Sabers).

So for Ninja Steel, having the deluxe roleplay be the deluxe morpher would eliminate one of their top revenue items. So, with the help of Saban Brands, they've chosen to make the Ninja Star weapon, which isn't used much in Ninninger after the first 15 episodes or so, to be the morpher and keep the sword just a sword.

As for the "cheeseburger morpher", they have yet to reveal their plans on that. My personal belief is that they will simply repaint the morpher in to blue/gold and use that instead.

The foam disc gimmick is for child safety. Last thing Bandai wants/needs are kids firing plastic shooting stars at each other. One hurt kid instantly multi-million dollar lawsuit.