It's probably because they are sitting so close they are touching. Most straight men won't do that. I mean that is what i've experienced. Most straight men i've known won't hug each other. They give that hand shake, back slap combo. LOL!
It could be a regional difference or it could be a social difference. What I mean is I live in the south so maybe it's not a big deal in California or New York and by social I mean maybe among the "Hollywood" crowd it's not a big deal. I don't see why men are so afraid of touching each other. Women are allowed to hug, kiss each other on the cheek, go to the bathroom together, etc. but men just don't do those things. Why? I guess it's just the long ingrained differences between what is socially acceptable for men and women. I'm rambling but I think you get my point. ha ha