Sorority girl wearing fraternity letters?

Im dating a girl in a sorority who had a frat sweatshirt in her closet. When i asked her about it she said that she stole it as a "joke" between the two organizations. I then asked her why it was clean and hanging up she said, because she would wear it. Im not greek and i know if i was dating a non greek girl who was weraing another guys hoodie i wouldnt like it. So whats up with that? Is it a normal thing with greeks?