I suspect it's because these motors are designed to be burned as a monolithic grain, but that presents shipping difficulties. So, they came up with a method of cutting the grains apart, then having the user reassemble them into a monolithic grain.
The one long-burn I have requires not only bonding the grains to the liner, but also to each other, which, to me, implies that they also want to inhibit the ends from burning.
-Kevin