Festivals cover a wide range, from those which are primarily focussed on concerts to those which are purely sessions. Most fall somewhere in between.

I don't think people who go to festivals only to participate in sessions are freeloaders. They contribute to the atmosphere of the festival, and in most cases if there were required to buy a ticket they would simply go elsewhere, so the argument that the festival is losing out doesn't hold water.

On the other hand, the festival is the catalyst which brings people together for the sessions, so it would be churlish not to acknowledge that by making some contribution, whether by buying a ticket or a donation. However I think it should be entirely voluntary.

I think this may be particularly true of small festivals where a few ticket sales may make quite a difference to financial success or failure. If all they succeed in doing is attract large crowds of sessioners who don't make a financial contribution then they might not happen again.