A regular session is, effectively, a club; a pre-existing, defined social unit. There are generally unwritten rules of one kind or another. They are usually not too difficult to twig. A newcomer is, naturally, expected to fit in with what is already happening. That is the same in any social situation anywhere. You begin by going along with the way things are done; if you want to introduce new ways, you must first gain acceptance on the existing group's terms. Nothing unusual in that; it's the way normal human interaction operates.

The discussion invoked at the beginning of this thread is not untypical of a certain class of musician, but it's scarcely interesting or surprising. People start such discussions quite regularly (there have been a great many here in the past) and little or nothing is ever said that is new. The distinction apparently being made there between "Irish" and "English" sessions does seem to be based on a fundamental misunderstanding of the natures of both: I can only think that those people, whoever they are, are using "English" to apply to the kind of generalist, mixed session that includes instrumental music and both traditional and modern song, and "Irish" to the tune-driven kind; my own experience (admittedly only going back about twenty years) would not support that viewpoint.

They all seem to live in the South, however; perhaps things are different there.