Wow, some strange arguments about this.
Here's the deal: the original god of Death, a MASSIVE evil #%^* had control of how the afterlife works. The only way to avoid that was to have a patron deity that would claim your soul before you went to the City of the Dead. Once you got there, you were judged by the god of the Dead, who decided either you were false to your patron god (where you suffered endless torture for failing to appease your god) or you were faithless, lacking any patron at all (where your soul ended up decaying as part of the wall).
Kellemvor attempted to make changes when he (the first non-evil god of Death), because as he was once mortal, he felt the system unfair (as many here do). However, the multiverse is a structured place, and rules cannot be easily changed, as Kellemvor was censured for his actions and forced to return to the original system. Not all of the gods like the system, but that is the system of the cosmos. It does cause almost everyone to pick a patron deity (most would have done so anyway) and encourages them to actually follow their deity with more than just lip service. FYI, you can give prayers and offerings to deities other than your patron, so you're not henotheistic (for example, offerings to Umberlee before a sea voyage is always wise).
As for the author's decision to create this... I don't know. Ed probably wrote a story about it when he was younger, set in the Realms before it was a D&D setting. It is official canon for the Realms, but the great thing about D&D is that you can change whatever you want as the DM anyway. The odd thing is that it really has about zero impact on the game at all, since you stay on the Fugue Plain long enough for most spells to bring you back anyway, except for things that can go back years/decades/centuries (which never really matter IME anyway).