No matter what the session is about, it comes down to manners. There are those who don't take the time to try to figure out the social environment they're in and just barge in and do whatever they want. If they're unwilling to figure out (observe or ask) the way a particular session operates, they're never going to fit in very well, and there are some who don't think they should have to.

In a mixed session, tune players can sometimes seize the moment and do tune after tune. Maybe only 3 or 4 people know the tunes, and the rest of the instrumentalists sit around waiting for something they know. In my experience, the occasional "show-off" piece or set is OK, but hogging the session isn't. (In other words, don't keep doing it.)

Likewise, songs in a session should be mainly of the participatory kind, with choruses or possibly room for tasteful accompaniment. A ballad or "performance piece" ("everybody listen while I sing") is occasionally OK, but, as above, hogging the session is uncool.

Even the "no hogging the session" rule isn't written in stone. Certain guest singers/instrumentalists have been invited to "hog the session" here, and we've enjoyed them.

Noodling, or whatever you call it - playing tunes to yourself - is annoying. Not that I haven't done it myself, mind you, but I usually notice little hints like dirty looks and the occasional "will you please stop that!?" comment. Singers can't find keys, and instrumentalists who would like to start another tune can't get the tune in their heads.

If it's a mixed session, realise there are tunes not intended for singing and some songs you shouldn't play along with. If it offends you to have to put your instrument down and sing for 5 minutes, you're in the wrong session. Likewise singers who can't tolerate sets of tunes. (If I say they should all bring spoons or buy bones or bodhrans and join in the fun, someone will threaten my life.)

It has been my experience that singers get treated rudely more often than instrumentalists. It often comes down to one singer and their song vs. an army of diddly players. Most of the instrumentalists are polite, but all it takes is one or two oblivious or downright rude folks to stomp on a song. I can't recall the reverse (singers stomping on tunes) ever happening, although I'll admit that people seem more likely to talk during tunes than during songs.

It comes down to manners...and tolerance.
If you can't stand the way a session operates, you can either try to change it to be the way you want, or you can get over it and enjoy the things you like. If you can't find a way to enjoy the session, maybe you shouldn't be there.

Just my opinion, and I sing acapella and play fiddle.