Ha ha!
Since we're having a Quote War (all in good fun!), you forgot to quote this "context" quote from the beginning of Hellfire Warlock in Fiendish Codex II:
“It is not the weapon that is evil, but the wielder.”
—Galena Todrick, hellfi re warlock
That said, it is absolutely correct that
3.5e sources do not require an Evil Alignment to become a Hellfire Warlock. So by all accounts, that should change on BGTSCC.
However, I can understand that the Evil Req for BGTSCC comes from an attempt to "keep the RP" within certain bounds that might otherwise be impervious to the literal Snowflake in Hell syndrome:
PLAYING A HELLFIRE WARLOCK
You walk a dangerous path. Each use of hellfi re brings you ever closer to death. You must be vigilant in your resolve and resist using this terrible magic recklessly. Since you wield the power of Hell itself, others are suspicious at best and downright hostile at worst. They fail to understand why you would use such power willingly and how you can resist its allure.
And more:
HELLFIRE WARLOCKS IN THE WORLD
“The offering of hellfire is yet one more way the fiends have worked their claws into our world. Each mortal who thinks to master this fell power is yet one more fool offering his soul to the Lord of the Eighth.”—Alód the Traveler, planeshifter
Most hellfire warlocks whom the PCs encounter have likely already succumbed to the temptations of evil. Usually min-ions in the service of an infernal cult, they lead groups of thugs or devils against good organizations or to hunt down and destroy adventurers. Only in the rarest circumstances does a hellfire warlock resist these temptations and use her power to advance the cause of good.
Cause, you see, and I know I'm going to get skewered for this, but it is the TENDENCY for Players to choose Snowflakeism, and I am no different! Yet in DnD as it was designed, there is a World—one and ours being the Forgotten Realms—where there are actually rare things, rare Beings, rare occurrences. On the BGTSCC Server World, "rare" is so utterly relative and often not-the-case, that it is only through OOC imposed limitations will we, the Players, get even the hint of rarity in our RP experience.
Since hellfire warlocks are nearly universally reviled, these characters keep to themselves. Their isolation pushes them to test the limits of their morality, and in time, most succumb to the darkness blooming within them. Hellfire warlocks are jealous and petty. They see others who practice their dark art as threats to their prominence and power, so these characters are swift to destroy their rivals.
Jealous and petty? Neutral Evil. "...destroy their rivals?" Does that sound Chaotic Good to you? Chaotic, yes, but...Good?
NPC Reactions
Even those hellfire warlocks who shy away from evil in the face of such wickedness meet scorn and derision in the world. Warlocks are generally not trusted, and those who willingly use the tools of evil for any reason are tantamount to the fiends themselves in the minds of most folk. Hence, most hellfire warlocks conceal their natures, using their infernal powers only when forced. So long as a hellfire warlock maintains her anonymity, she finds others are indifferent. Those who become aware of her capabilities immediately become unfriendly, seeing her as a menace. This animosity extends even to evil creatures, which are often unsure what to make of such a curious character—or see her as a rival to be destroyed.
I mean, I also enjoy playing the loner PC that just grinds and loots with the rest of the playerbase, but...come on! Chaotic Good Warlock PC: just waiting for the Halloween Costume Party at House Darius so they can actually "hang out" with their IC friends...in public! LOL. Yes, I'm being a sarcastic shit here, but that's how Snowflakeism goes on BGTSCC. I should know...I'm just as guilty as anyone (I have an Aasimar Warlock corrupted by Glasya and they burned and scorned by her abuse, and know channels his dark powers to destroy any power that He deems greater than his own...all the while being able to supernaturally cast the Light spell, the basic "Good" spell!!

)
HELLFIRE WARLOCKS IN THE GAME
A hellfire warlock is a natural ex-tension of the warlock class. One can be a new agent in the armies of the Nine Hells or an established occultists who dabbles in the forbidden. Hellfire warlocks are particularly attractive to players who enjoy playing antiheroes. This prestige class grants access to otherwise forbidden abilities. This class is also specifically intended for the warlock class, enabling you to expand this potent class in new directions.
So, all this is just my opinion. But Rules are Rules and the Core Sourcebooks absolutely support non-Evil Warlocks harnessing Hellfire. So break down the wall Mr. Hellfirechov!!!