First let me say I think this is sad, but not isolated. I am the leader at the "other" school that has taken in girls that were not "allowed" to join or only wanted to join GS if they could go to a different troop. While you all brought up great points, I want to point out another side of this issue and point out that not all leaders who turn away girls are ugly green heartless monsters.
First, your job as being a leader depends highly on your parents. They are the supporters, drivers, family cookie coordinators, fellow campers, etc. Picture yourself for a moment, your own family responsibilities put to the side, feeling overwhelmed, and you just got your registration paperwork in turned in and have done your "happy dance" because it is done for another year. Then, someone with good intentions sends out an "invite" to join the troop and the one girl who wants to join you know her mom will cause more work, frustration, and you now have to re-submit your paperwork. Not an ideal reason but we all have our limits.
Second, there are times you have to stand yoru ground. A couple of years ago that I had come to the VERY difficult decision not to accept for the new year, the only reason I did not have to go through with it is because she dropped from the troop. My decision was primarily based upon the mom's behavior. She was always late, needed special treatment (in my opinion just for attention), brought her son who was out of control to the meetings after repeatedly requested not to, and verbally abused her daughter in my presence whithout any cause. While I felt that scouts to this one girl was for the most part good, although I hope sitting at home with the TV was less confrontational with Mom. It came down to protecting the other girls in the troop, although I did report the situation to the authorities out of concern for this girl.
Putting "restrictions" on leaders would loose leaders and then effect the girls. If I were in this troop I would first talk to the leader and offer to serve as a "buffer" with the mom and help ease the obvious tentions, or go start my own troop.