Human lice can't live on things other than humans for more than 48 hours which I think might be what you are thinking of.
Guinea pigs get guinea pig lice, just like other humans get human lice from other humans. Your guinea pig can't give you lice and you can't give your guinea pig lice. Although the lice could live ON you for 48 hours or the other way around.
They probably don't want to let the guinea pigs leave until they are positive the problem has been fully treated, which actually is a good deal for you because you wont have to deal with that either. Like other people have stated, lice would be killed by treatment but then you have to get all of the eggs as well.
I think it is a good sign that they are doing that. They are ensuring the animal has been treated for any problems and that the treatment has been fully effective/administered in the proper way at the proper times. And I believe, someone correct me if I am wrong, but the same treatment is used for lice as is for mites anyway but they are two different things.