IT just comes from a simple reading of the card and making sure it says what you think it says. Often I think a card says something and then when I re-read it, I find that it does not say what I think it says.
Some cards limit when a LS can be negated and when it cannot. For example.. "Cannot be negated if played on a Genesis Character" clearly limits when a card can be negated and when it cannot.
In the case of this LS it says:
Jeremiah 11_8 (RA)
Type: Lost Soul • Brigade: None • Ability: None • Class: None • Special Ability: Unless a Covenant is in play, the first evil Enhancement you play each battle cannot be prevented by a good card. • Identifiers: None • Verse: Jeremiah 11:8 • Availability: Rock of Ages booster packs (None)
So if CAN be prevented by an evil card (such as playing an enhancement on KoT) or a neutral card, like a fortress, site or LS.
In this case, the FBtN LS would negate the ability on the CBP LS and render it just like a LS with no SA until the other soul is rescued, buried or negated.