A lot could be said about this and I have been trying hard not to get into this but a few fairly random thoughts:

I think there's a differentiation between being encouraging to learners and being open to all comers. It can be a fine line but there's a distinction none the less.

The way I see it, a session is a meeting of like minded musicians. If you're out for a few tunes with friends it can be very disconcerting if someone joins out of the blue who is not sensitive to the dinamic of the situation and the music being played, yet expecting to be fully accommodated. Not all comers are sensitive to the social situation that a session is and often they can be the loudest in denouncing a session as being 'elitist' or 'unfriendly'.

I don't particularly fancy playing in noisy pubs (although a quiet one can be fine) and would prefer to play in more private situations. But when deciding whether or not a session is 'real' I think a few things should be taken into account: first of all, musicians not paid to be in a particular spot at a particular time may find something else to do on a night, or find themselves not in the mood. In other words, they won't turn up every time but only when they feel like doing so. Which is fine but won't be appreciated by a publican who wants to fill the house and who has a 'music tonight' sign outside. Let alone people on forums like this who travel to a place expecting a 'session'. Also, if you look at a session as a meeting of like minded musicians, playing for the sake of playing music they enjoy, it's not hard to understand why they wouldn't want to advertise the time and place of their meeting to the general public.

And for what it is worth, I wouldn't expect to walk into a place and join a group of musicians without being invited to do so. But then, YMMV and 'real' sessions can come in many different guises.