The fact people play in a public place does not mean it's a free for all. I would never presume to be welcome to join when I see people I don't know play 'a session'. It's just not right to barge in a sit down like you own the place. Certainly never expect to be made welcome if you're going to bring the level of proceedings down.

The 'it's just a session, nobody is listening anyway' is just an appalling bit of attitude, if you're not playing for the people around you, at least play for your own enjoyment and listen to eachother to produce something coherent and pleasing. If you just want to be in the same room as other people and make noises, maybe invite some friends over to your home but don't take that out to spoil other people's night out.

There's nothing wrong with not playing all the time when at a session. If you don't know the tune, maybe shut it for a moment and listen. Ditto if you have nothing to add to the mix.

That's my take on it anyway.