Thanks for the great responses! Some really interesting points have been brought up.
My reservation was not whether or not they deserved a good home - I agree 100% that all guinea pigs deserve great homes, regardless of their background! And of course it's not their fault that they've ended up where they have. I just really don't want to condone "throwing" guinea pigs away when you're done with them.
Saying that, I would have no hesitation adopting a guinea pig from rescue who came from this background, and I didn't really consider that so many probably do go straight to rescue, so by adopting a piggy straight from the breeder, it could just be cutting out the middle man, as Laura said!
I've sent another email asking for any additional information on the girls (and mentioning that they'd join my sow), so we'll see how it goes.
Thanks for all the opinions!