Me neither. (As in, also don't understand why she would have to drop out of Girl Scouts).
Have you spoken with the Girl Scout leader? She might have some good ideas for you. Maybe she can help figure out good ideas for discipline.
A friend of mine has a daughter a little younger than yours, age 7 right now. Just recently she was talking about how this daughter gave away some of her dolls to other friends - and then regretted it, because they were her favorite dolls. It seems to be common to this age group to give things away, to exchange toys and even clothes with friends. And then sometimes it sort of backfires, when the child realizes what she has done.
Maybe giving the money away was just part of this developmental stage, and she didn't really realize why it was wrong. Or got carried away and then realized it.
I wouldn't spank. That creates antagonism and pain and resentment, and I'm sure those aren't the lessons you want her to learn.
Let her know how very, very disappointed you are and let her know why this was very wrong. You might have her do some extra jobs - I'm sure there are plenty around the house and maybe even at the chicken coops! - to earn back the money. Maybe that will help her make the connection that people work hard for that money, and her own labor can earn money, too, and she would not like it if that money was taken from her.
And for you, now you know that she is not ready to handle that much money on her own. That's not a crime, that's just part of being 8 years old. What she learns now can help her learn more responsibility, and that's where you want to be going, in the end, with all this.