It is designed to stop girls getting unsolicited pervy messages from guys. They keep their profile private. An example was my sister. Within the first week of her being here, she was flooded with unwanted attention. Her profile now says something like "don't bother messaging me about love and sex. You won't get anywhere".
No, I don't think all people have that restrictions. I think it's not something that GAG has set, if I am not wrong that restriction can be set via your profile. I mean you can set if you only want to receive messages from the people you follow and trust and not from everyone as such. Hence it's like a security, it gives them a sense of security.