Sessions are for musicians. If they happen to occur in a pub (as they usually do) so be it. But the music is essentially for the musicians and the punters in the pub are secondary to the session. If they are there, they can enjoy it. It is a common fact that sessions do tend to be fairly introspective - a happening amongst musicians; what goes on around the session is usually fairly irrelevant.

Now that can sometimes be read as unfriendliness - I don't believe it is intended to be.