We like to hug girls we really like, because it's nice. We like that person, and hugging them feels wonderful, especially if they hug back. I do it with my best friend, because I love her and would do anything for her, but I'm not interested in something else.
But hugging from the back, that's different. Especially the kiss on the cheek. I'd feel like I'm crossing the lines if she isn't my girl.
If I see a guy hugging a girl from behind and kissing someone who isn't his girlfriend, I'd be 100% he thinks that girl is hot and would sleep with her without thinking twice. It doesn't necessarily mean he would, but that sh*t isn't cool to do with someone who isn't your girlfriend, because I know if that girl ever does get a boyfriend, that guy isn't going to put up with that sort of sh*t coming from one of her guy friends.
My best friend has a boyfriend, who doesn't mind that me and her are friends, or that we hug each other. But I'm pretty sure that if him or any self-respecting guy saw me hug their girlfriend from behind, and kiss her on the cheek, things would get pretty heated.