Seems to be a lot of mixed messages going on here.
My two pennies.
1. Session/Singaround. Well, my main experience is Sidmouth. Anchor for songs, Radway for tunes, Bedford for a bit of a mix of both. All three are not lead by anyone, except for the stick tradition for the songs. Which makes sense really, 8 different songs being sung at the same time might start to resemble Stockhausen!
2. Open Mics. Normally half organised, name on a list type of thing, Small PA, bit like a singers night at a folk club.
3. In a town based festival, some pubs may take advantage of all the possible extra clients in town, to put on non folk events if they've got the space, to get local bands to play....(Not organised by the festival)
4. Freeloaders? Well having been booked as a paid artist many times at Sidmouth in the past, I still enjoy going, Pay for my own B&B (Not cheap) and if I fancy a concert, I'll buy a ticket. I go, mainly to see old friends and play with them, I don't think that's freeloading? Surely it all just adds to the general festive mood.
5, Marquee style festivals (Towersey for instance) They have a day ticket policy, You can get on site for a tenner, and enjoy the stalls food, ambience, etc, and normally at some point an impromptu session will kick off in the bar. There was a cracking one last year, went on for about 5 hours, and some pretty well known booked performers popped in for a pint and a tune. Not bad for a tenner.
Obviously all festivals are different, depending on their situation/location. I think it's just a case of doing a bit of research before deciding where to go. As a non singer, I wouldn't expect to go to an all singing festival and start playing lots of tunes would I?