I like both tunes and songs (because I do a bit of both)and find it hard to understand why some people get so heated when the "wrong" sort of music predominates in an open session. But if I have to find reasons, I'd say:

Some singers dislike pure tune sessions because after an hour or two of tunes you're just bursting to sing if you don't get the chance. This is bad enough if you also play an instrument but must be worse if you don't and have no way of joining in the music. Some singers also find it difficult to cope with the "free-for-all" etiquette at tune sessions, and find it rude if no one asks them for a song - this is a misunderstanding on their part because they're used to song circles and the like, where people take turns or perform.

And then there are instrumentalists who don't like songs . These are generally the players who don't sing themselves, and don't get the point of songs. They feel they've come to play together, not to listen, and get restless if they have to sit quiet and pay attention.

The most accessible sort of singing to have at a mixed session is something that will stand a bit of improvised accompaniment, sung in a convenient key. Or, failing that, rousing choruses or shanties that allow people to join in. Some sessions will provide a quiet, attentive atmosphere to listen to an unaccompanied singer, but some won't. If singers were more aware of this, they might find sessions more welcoming.

To my mind, the easiest atmosphere at mixed sessions is when most of those who sing are also instrumentalists. But if singers really don't like tunes, they're probably best to stick to song clubs.

Marje