You found out, expressed your disappointment and grounded her. She doesn't think it's fair, so you listen to her reasoning behind the fairness issue. Her explanation makes no sense to you. You shouldn't be.
Teens think and reason differently than adults.
In their world of fuzzy justification, often their explanations make perfect sense to them. You do this and this will be the consequences. It makes perfect sense to us. It's important to remember that in their inexperience, teens don't see the world as we do. We have been around long enough to hear the stories about the rapes, the murders, the hidden dangers lying in wait for them.