No it's not. There is literally no difference in the fiction between an NPC and a PC, so treating them differently makes no sense. If it's bad for one to be treated like they are all the same by the thousands, it's also wrong for the other.
That doesn't really explain it. If thousands of people advancing in exactly the same manner is an issue, NPC or PC doesn't change that. It's not a problem that is specific to either category.
"Divine magic, as the name suggests, is the power of the gods, flowing from them into the world. Clerics are conduits for that power, manifesting it as miraculous effect. The gods don't grant this power to everyone who seeks it, but only to those chosen to fulfill a high calling."
You really...
Yet for some reason it's not an issue for the thousands of PCs that receive the same training and same growth in abilities in exactly the same order. Why are NPCs different?
I think that's more because those books don't use NPCs that join the party as an equal. The 5e DMG suggested PC class levels for NPCs who were going to be party members.
I'm of the opinion that what's good for the goose, is good for the gander. If NPCs can do it, PCs should be able to do it as well. If PCs can do it, NPCs should be able to do it as well.
To really engage with what you are describing, you'd need a classless system, and D&D isn't that kind of...
They clearly don't in 5.5e, because an archmage wizard NPC has different hit points, abilities, etc. than a PC wizard. Both are wizards. The laws of the world are different for them.
The laws SHOULD be the same, though.
Then that comes into play with Specific Beats general and they choose those options. Until then, they've stupidly made a pact with some unknown entity and don't find out about who or what it is until later. That's the fiction.
In 5e there are NPCs that do have them. I believe they took it out for 5.5e, but there's no reason a DM can't or shouldn't make NPCs with class levels.
No. I'm saying the classes and levels are built to match the fiction. In the fiction the fledgling wannabe warlock contacts some unknown entity and blindly makes a pact. The rules call that level 1 warlock and give it the mechanic Level 1 Pact Magic.
The fiction and mechanics need to be in...