Here's what I'd hoped/imagined the scene looks like so far to a PC.
Beretta and Spline could indeed be former gunslingers. That they are somehow compelled, or committed, to helping you in your quest - because they, as ex-gunslingers, and having some magic about them, can indeed understand and know what you are doing, and they feel it is very important. Maybe the most important quest in the world.
They want to help you, but in the face of the party's distrust, feel like they aren't able to help you as they need to. So they are trying everything they can think of to show that they are trustworthy.
When Mattie, however, didn't want to enter into the magic they offered, they feel the magic is less useful. But they already offered it, and don't want to take the offer away if it WOULD be useful for Henry and maybe Rose.
If anyone joins them in the magic, then Spline looses a lot, and the party members who join them end up knowing a dark secret of each other. Beretta isn't happy about this, but if it helps the party, she will do almost anything (including Spline taking all this harm) to help you.
I agree, it doesn't feel like a good deal - but Roland makes a lot of decisions that feel like a bad deal in the books, so I leave it to the PCs to decide whether they want in on this strange offer.