Every pub and/or session venue is different too. The session may be in a lounge or separate room and the punters don't necessarily have to suffer it.
So, yes, there is scope for beginners' sessions and even the use of sheet music, song sheets and so on.
This doesn't happen in Bell's or most of the regular sessions, of course.
In some bars, musicians are encouraged to play as the music is regarded as an attraction. In some other places, the pubs may be doing the musicians a favour by allowing them to play. Circumstances vary.